﻿ ﻿Drill Answers and Explanations - Cracking the SAT Chemistry Subject Test

## Cracking the SAT Chemistry Subject Test

Part III

Chapter 3 Some Basic Stuff

1 A If you chose “mass” or “density” just because those words appeared in the question, you fell for the temptation trap. The test writers wanted you to do this. To answer this question correctly, you have to think carefully about what the words mean, and you must not rush to an answer simply because an answer choice contains a word that reminds you of something.

Let's work through this. If you're given a sample of some substance, and you know the density of the substance, what additional information would allow you to calculate the mass? If you know its density, then you know the ratio of mass/volume. Therefore, if you were also provided with the sample's volume, you could figure out its mass. That's why (A) is right.

2 E You're asked to determine which quantity or property always varies with the number of molecules in a given sample of a substance. You have the knowledge to answer this question, so don't let the wording throw you.

Think about what you know; mass is a measure of the quantity of matter. This means that for any sample of a particular substance, mass always varies with the number of molecules in the sample. More molecules of substance X have a greater mass than fewer molecules of substance X.

What about volume, temperature, density, and pressure? Volume, (A), is a measure of the space a sample occupies. The volume of a substance might vary as the number of molecules of that substance is varied; however, it does not have to change with the number of molecules. This is certainly true in the case of gases: A rigid container (with a fixed volume) may hold vastly different amounts of the same gas because gases are expandable and compressible.

Changing the amount of a substance will not raise its temperature (B). By remembering the formula for density, (C), (d = m/v), we see that the density of a substance will remain the same despite an increase or decrease in the mass of a substance, if the volume is changed in direct proportion to the change in mass. The pressure, (D), exerted on a solid or liquid will remain the same, regardless of the number of molecules of the substance. The pressure of a gas in a closed container will change with an increase or decrease in mass, but if the volume of the container holding the gas is increased or decreased as the mass changes, the pressure will not change.

3 C Kilograms are used to represent mass, and liters are used to represent volume. Mass/volume = density, so (C) is correct.

4 B If you remembered to associate temperature with average kinetic energy, you knew that (B) was correct.

103 T, F Divide and conquer! First, examine each statement separately, and determine whether it's true or false.

Is the first statement true? Yes, it is, although camouflage might at first prevent you from realizing that. But because

density = simple algebra allows you to calculate mass, if you know density and volume.

mass = (density)(volume)

Now look at statement II, and decide whether it's true or false. What does the statement mean? For a bigger piece of some substance, the relationship between mass and volume is different than it is for a smaller piece of that same substance. That's false. For any substance, the relationship between mass and volume is given by the density of the substance. For solids and liquids, density does not change with sample size.

104 F, T Again, divide and conquer. Look at the first statement. Is it true? For solids and liquids, volume can increase somewhat by heating the substance, even while the mass remains constant. The volume of a constant amount of gas can also be increased by heating or cooling the gas, or by simply placing the gas in a larger container. Therefore, the statement is false.

Look at the second statement. Does density represent mass per volume? Yes, it does. The second statement is true. Since one of the statements is false, you leave the CE oval blank.

105 F, F Divide and conquer. Since we do not know the temperature of X or Y, we cannot say which 10-g sample has more heat content, so statement I is false.

Consider statement II. A substance with a relatively low specific heat will undergo a relatively large temperature change upon the addition of heat, so statement II is false. Don't fill in the CE oval.

26 E Don't even look at the answer options until you really understand the situation that's being described. You've got two objects. They occupy the same volume, but one has half the density of the other. Since

density = you know that this means that the object with less density has one-half the mass of the object with greater density.

Object A has twice the density, so it should have twice the mass of an equal volume of object B. That's why (E) is right.

33 C Remember q = mcT? Substituting the values into it gives

30 cal = (15 g)(0.5 cal/g · °C) ∆T

Solve for ∆T, and you'll get 4°C. This is the increase in temperature. So if the substance was at 30°C, it's now at 34°C. That's choice C.

38 E Adding more oxygen gas will certainly increase the mass of the gas sample; therefore, statement I is true.

Since density is the ratio of mass per volume, increasing the mass while maintaining the same volume will increase the density of the gas, so statement II is also true.

Pressure is a measure of the force per unit area with which gas molecules collide with the walls of the vessel. More gas occupying the same volume will mean more collisions and, therefore, greater pressure. So statements I, II, and III are true, and (E) is the answer.

Notice that even if you don't know if all of the statements are true, you can make a really good guess with just a little knowledge. Just knowing that statement I is true allows you to eliminate choices B and D, and by knowing that statement II is also true, you can rule out choice A. At this point, you have a 50 percent chance of choosing the right answer even if you know nothing about pressure!

Chapter 4 Atoms: The Building Blocks of Matter

4 A Helium is an element, and the smallest piece of an element is an atom. That's why (A) is correct. An atom is the smallest particle of an element that still retains the properties of that element.

5 E You know that every electron carries a charge of –1, and that an atom becomes a positively charged ion when it loses an electron(s), and a negatively charged ion when it gains an electron. That's why (E) is correct.

6 D Which of the components of the nucleus—;protons or neutrons—;have a positive charge? Protons have a positive charge, while neutrons have no charge—;they are electrically neutral. The answer is (D).

7 C Don't be fooled by the mention of uranium; it's just another element. You know that isotopes of the same element do not differ in their number of protons, may differ in their number of electrons, but must differ in their number of neutrons. That's why (C) is correct.

8 D The atomic number of an element depends on the number of protons in that element's atoms, so the answer must be (D).

105 T, T, CE Divide and conquer! Evaluate the first statement without looking at the second, and decide whether it's true or false. It's true. The periodic table reports atomic weights but not mass numbers.

Now see if the second statement is true or false. Is it true that a mass number can be assigned only to a single isotope of an element but not to an element in general? Yes. Mass number = number of protons + number of neutrons. Different atoms of the same element may vary as to the number of neutrons they contain. That's what makes them isotopes.

Both statements are true. Now see if this sentence makes sense: “The periodic table does not report mass numbers because a mass number can be assigned to one isotope of an element but not to an element in general.” It does, so fill in the oval marked CE.

106 F, T Divide and conquer. Look at the first statement on its own, and decide whether it's true or false. Electrons are negatively charged, so if we add an electron to an atom, it becomes a negatively charged ion. This statement is false. Now look at the second statement; is it true? Yes. Electrons are negatively charged. The first statement is false, and the second is true. Do not fill in the CE oval.

27 E You know the answer, so don't fall into the camouflage trap. The atomic number represents the number of protons in the nucleus, and it gives the atom its identity. Any sodium atom or ion must have, in its nucleus, the same number of protons as any other sodium atom; otherwise it isn't sodium. Both (A) and (B) are wrong because sodium ions can carry different charges, depending on how many electrons they've gained or lost. (C) and (D) are wrong because different isotopes of sodium will differ in the number of neutrons in the nucleus. That means their mass number will differ. But all sodium atoms or ions must have 11 protons in their nuclei. That's why (E) is correct.

28 A You know what isotopes are—;atoms of the same element that differ in their number of neutrons. Isotopes may also differ in their number of electrons. They must differ in their mass numbers since they have different numbers of neutrons. They can't differ in their number of protons, or they wouldn't be atoms of the same element. (B), (C), (D), and (E) are incorrect, and (A) is correct.

Chapter 5 Chemical reactions and Stoichiometry

6 D A diatomic molecule consists of two atoms that are bonded. Among the choices, only nitrogen monoxide (NO) contains just two atoms. Never mind that the atoms are from different elements; it's still a diatomic molecule. The correct answer is (D).

7 E First, eliminate the obvious wrong choices: By inspection, you know that N2O and NO are nowhere near 108 amu, so cross them out. Then you'll need to do some math. Use the periodic table to find the atomic weight of each element in the compounds. Then multiply this mass by the number of that kind of atom in the molecule. Add the mass contributions from each element to get the formula weight. When you do this for choice E, you'll get
2(14 amu) + 5(16 amu) = 28 amu + 80 amu = 108 amu. (E) is your answer.

8 B Look at the ratios between different types of atoms. If the ratio can be put in terms of smaller whole numbers, then the formula is not an empirical formula. Check out choice (B). In this molecule the ratio of carbon to hydrogen to oxygen is 6:12:6. If we divide this ratio by 6, we'll get the simpler (though still equivalent) ratio of 1:2:1. So the empirical formula of C6H12O6 is CH2O. Since the molecular and empirical formulas differ, this must be the answer.

9 C Proceed as you did in question 7. If you are comfortable with your intuitive sense of relative molecular weights, you should be able to eliminate choices (A) and (E) without doing any calculations. After you get the formula weight for each choice, compare it to the weight contributed by oxygen. Look at choice (C). The formula weight of SO3 is 32 amu + 3(16 amu) = 80 amu. Oxygen's percent, by mass, is × 100% or 60%. (C) is correct.

102 T, F Divide and conquer! Look at the first statement. Is it true or false? Of course it's true. Chlorine is an element. What about statement II? It's false. Remember that chlorine exists in diatomic form at room temperature and atmospheric pressure.

103 T, T, CE Divide and conquer. Consider statement I. One mole of HBr contains the same number of molecules as one mole of NO2—;6.02 × 1023 molecules—;but this doesn't necessarily mean that 6.02 × 1023 HBr molecules weigh the same as 6.02 × 1023 NO2 molecules. Do ten paper clips weigh the same as ten elephants? Certainly not. Use the periodic table. One molecule of HBr has a mass of about 1 amu + 80 amu, or 81 amu. One molecule of NO2 has a mass of about 14 amu + 2(16 amu), or 46 amu, so one molecule of HBr has greater mass than one molecule of NO2. And one mole of HBr would thus have greater mass than one mole of NO2. Statement I is true.

What about statement II? Well, we've already determined that it's also true.

Now does this sentence make sense? “One mole of HBr has greater mass than one mole of NO2 because the mass of a molecule of HBr is greater than the mass of a molecule of NO2.” It sure does, so fill in the oval marked CE.

27 B Remember that to calculate the formula weight of Ca(NO3)2, all you need to do is add up the weights of its constituent atoms. Calcium's atomic weight is 40 amu, nitrogen's is 14 amu, and oxygen's is 16 amu. That means the formula weight = 40 + 2(14 + 3(16)) = 40 + 2(14 + 48) = 40 + 28 + 96 = 164 amu. (B) is correct.

48 D You're given a substance's mass composition, and you need to determine its empirical formula. So imagine that you have 100 g of the substance.

• You've got 9 g of magnesium, and magnesium's atomic weight is 24 amu, so you have = 0.375 mole of magnesium atoms.

• You've got 91 g of iodine, and iodine's atomic weight is 127 amu, so you have = 0.717 mole of iodine.

• The ratio of iodine atoms to magnesium atoms is , which is very close to 2:1.

This means that the empirical formula of the substance is MgI2, and that's why (D) is correct.

6 A You know to associate Gibbs free energy with the spontaneity of reactions—;that alone is enough to tell you which answer choice is correct. But if you want to take it further, remember that a reaction is spontaneous if the overall combination of enthalpy change—;energy change—;and entropy change is energetically adequate. Even an endothermic reaction—;a reaction in which energy is consumed—;can proceed spontaneously if it's accompanied by a large enough increase in entropy.

7 C You should associate enthalpy with the words “exothermic” and “endothermic.” Exothermic reactions release energy (which the universe tends to like), while endothermic reactions consume energy (which the universe tends to dislike). If, in the course of the reaction, the enthalpy change is negative, the reaction is exothermic. If it's positive, the reaction is endothermic. That's why (C) is correct.

8 D Remember to always associate entropy with disorder. That's what it measures, and that's why (D) is correct.

105 F, T Divide and conquer! Look at the first statement on its own. Is it true? No. The fact that a reaction is exothermic does not necessarily mean that it's spontaneous. The first statement is false.

Does the universe favor a negative enthalpy change? Yes. Generally speaking, it likes exothermic reactions. So the first statement is false, and the second is true.

106 T, T, CE Divide and conquer. Evaluate the first statement by itself. Is it true or false? It's true! In order for ice to melt, it must absorb heat. Melting involves a net absorption of heat energy because bonds between water molecules must be broken in order for melting to occur; it is an endothermic process.

What about statement II? Your own experience tells you that this is true. Ice must absorb heat in order to melt. Now put both statements together: “Ice melting is an endothermic process because heat must be absorbed by ice if it is to melt.” Does it make sense? Absolutely. So fill in the CE oval.

28 C Starting with (A), let's plug in choices. If the coefficient for O2 is 1, then there are two oxygen atoms on the left. Since we are starting with 3 oxygen atoms on the right, this number is too small. Try (B). If the coefficient for O2 is 2, we get 4 oxygen atoms on the left. Putting a 2 in front of H2O gives us 4 oxygen atoms on the right. However, now we cannot balance carbon without upsetting the oxygen balance. So (B) is also wrong. What about (C)? If the coefficient for O2 is 3, then we have 6 oxygen atoms on the left. Putting a 2 in front of CO2 and H2O gives 6 oxygen atoms on the right. So far, so good. Notice that putting a “1” in front of C2H4 puts carbon and hydrogen in balance, so the answer is (C).

50 A Notice that the consumption of 2 moles of Na releases 822 kJ of heat. What happens if only 0.5 mole of Na is consumed? Since 0.5 is only 25% of 2, only 25% of 822 kJ of heat will be released. As you can see, choice (A) is about one-quarter of 822 kJ, so the correct answer is (A).

56 B When information about more than one reactant is given, brace yourself for a limiting reactant question. The stoichiometric mole ratio of Al to Fe2O3 is 2:1. The atomic weight of 1 aluminum atom is roughly 27 amu, which means that 1 mole of Al has a mass of about 27 g, so 80 g of Al represents nearly 3 moles. The formula weight of Fe2O3 is 160 g, so we have approximately 0.5 mole of Fe2O3. We've calculated that the actual mole ratio of Al to Fe2O3 is roughly 3:0.5 (or 6:1). This means that Fe2O3 is the limiting reagent. Notice from the balanced equation that for every 1 mole of Fe2O3 consumed, 2 moles of Fe are produced. If 0.5 mole of Fe2O3 is consumed, then 1 mole of Fe is produced. The correct answer is (B).

Chapter 6 Electron Configurations and Radioactivity

9 B You may be tempted to pick (E), but don't. Atomic theory is associated with a fellow named Dalton and says that all elements are composed of atoms. For this test, you're not expected to fully understand the De Broglie hypothesis; you're just supposed to associate it with the idea that matter can be conceived of as waves, and waves can be conceived of as matter. So (B) is correct.

10 A You learned that Bohr incorrectly believed that electrons circled the nucleus in orbits, the way planets circle the sun. So (A) is correct.

11 C For this test, you need not completely understand the Heisenberg principle; you only need to associate it with the idea that one cannot, at any one moment, know both an electron's position and momentum. So (C) is correct.

105 T, T Divide and conquer! Evaluate the first statement on its own, and decide whether it's true or false. Is it true that according to the Bohr model electrons circle the nucleus in true orbits? Yes, it is.

Now, look at the second statement by itself. Is it true? Yes. Now let's find out if the sentence makes sense. “The Bohr model of the atom is inaccurate because an element may exist as several isotopes each with a different number of neutrons in the nucleus.” The second part of the statement has nothing to do with the first. Both statements are true, but they have nothing to do with each other, so do not fill in the CE oval.

106 F, T Evaluate the first statement by itself, and decide whether it's true or false. Is it true that krypton is an unstable atom? No, that isn't true. Krypton is a very stable atom. Why? Because it has a full octet in its outermost shell.

Now look at the second statement. Is it true or false? It's true: Atoms with 8 electrons in their outermost shell are stable. The first statement is false, and the second is true.

31 B Here you're given the configuration and asked to identify the element. The easiest way to solve this problem is to add up the electrons in the configuration; you'll find that their sum is 27. Since the question asks about an atom, and not an ion, this means the answer is Co, which has the atomic number 27. The correct answer is (B).

32 E Here you're given the element and asked to identify the electron configuration. Follow the steps we showed you for writing electron configurations, and you'll see that E is correct. If you had trouble writing the electron configuration, you could also have arrived at the answer by testing each answer choice. See which answer choices have superscripts that add up to the atomic number of Tc, which is 43. Then eliminate the other answer choices, and take your best guess.

33 C Do it the easy way: Count up the superscripts. They add up to 54. Look on the periodic table, and you'll see that element 54 is xenon. Xenon, as you can see from the table, is a noble gas.

14 D As we said earlier, the emission of gamma rays generally accompanies other forms of radioactive decay. So (D) is right.

15 A In order for its atomic number to be reduced by 2, an atom must lose 2 protons. In order for its mass to be reduced by 4, it must also lose a total of 4 nucleons (protons or neutrons). The atom loses 2 protons and 2 neutrons. That's the description of alpha decay, so (A) is correct.

16 B Beta decay—;which involves the emission of an electron—;converts a neutron into a proton. Positron emission and electron capture (a decay process that you don't need to know for the test) convert a proton into a neutron. Beta decay is the correct answer; that's choice (B).

107 T, F Divide and conquer. The first statement is true. The second statement is false.

108 T, T, CE Divide and conquer. The first statement is true: If an element decays for one half-life, half of the original sample remains. If it decays for two half-lives, one quarter of the original sample remains. The second statement is also true. Does the whole sentence make sense? Yes. So fill in the CE oval.

39 C In this problem, an atom of Rn (radon) with an atomic number of 86 and a mass number of 222 undergoes a change. What happens to it? All of a sudden it's a different element, Po (polonium), with an atomic number of 84 and a mass number of 218. It lost 2 protons and 2 neutrons. The Rn atom has undergone alpha decay. In fact, you can see that an alpha particle—;helium-4 nucleus—;has been emitted as a part of the whole process. We're dealing with alpha decay, and (C) is correct.

40 D To begin with, you can see that an electron has been emitted, so that's one way to know, right away, that we're dealing with beta decay. You can also see that the atom of I (iodine) has turned into something else—;xenon—;because its atomic number increased from 53 to 54. But its mass number stayed the same, so it looks as if a neutron was turned into a proton. That's beta decay, and (D) is correct.

Chapter 7 The Periodic Table and Bonding

11 B Remember that the group 2A elements are alkaline earth metals. Calcium (Ca) is in this family, so (B) is correct.

12 D Metals bond by losing valence, electrons, so eliminate A, B, and C. This leaves us with fluorine and neon.

They're both nonmetals; however, neon is a noble gas, so it generally does not form bonds. Fluorine does. (D) is the answer.

13 A The first ionization of an element refers to the amount of energy needed to remove one electron from an atom. The second ionization energy refers to the removal of a second electron. What kind of electrons are hardest to remove from an atom? Inner shell electrons. So the correct answer should be an element whose atoms have only 1 valence electron. Why? Because removing a second electron from such an atom would involve removing an inner shell electron and thus take an enormous amount of energy. So which of the choices has atoms with 1 valence electron? Think group 1A elements. Sodium is one of those, and (A) is correct.

109 T, T, CE Time to divide and conquer. Is the first statement true or false? It's true. What about statement II? It's also true. Inner shell electrons are held more closely to the positive nucleus than are valence electrons; inner shell electrons are held too strongly to be useful in bonding.

Let's see if the whole sentence makes sense: “Only an atom's valence electrons can participate in bonding because an atom's inner shell electrons are held too tightly to be shared or transferred.” It certainly does, so fill in CE oval.

110 T, F Consider statement I. Does potassium have greater metallic character than iron? Remember that metallic character involves the ease with which an element's atoms can give up electrons. Potassium is much more reactive than iron and thus can be expected to give up its electrons much more readily, so potassium does have greater metallic character, and statement I is true.

Look at the second statement. Is it true or false? It's false. Potassium is an active metal, and active metals tend to have lower melting points than transition metals such as iron.

39 A Don't be fooled by choice (E). Chlorine is very reactive, but it isn't a metal. Recall that the alkali metals are the most reactive metals. Sodium is a member of the alkali metal family, so (A) is correct.

42 D Nickel is a transition metal, so we can expect it to be malleable, ductile, and lustrous (shiny). As is true of many transition metals, nickel compounds are intensely colored (usually a bright green), so eliminate (A), (B), (C), and (E). That leaves (D). Nickel is a conductor of heat (and electricity), not an insulator. Pick (D).

51 C Remember the periodic table trends. As you move from left to right within a period, atoms get smaller. So the smallest atom of a period 4 element is krypton (Kr). Eliminate any choice that does not start with Kr; that makes (C) the correct answer.

14 C Metals are held together by metallic bonds. Potassium is the only pure metal listed; therefore, (C) is the correct answer.

15 D Ionic compounds possess ionic bonds. How can you spot an ionic compound by looking at formulas? Easily; just find a compound composed of a metal and a nonmetal. Choices (A), (B), and (E) are substances composed solely of nonmetals. Choice (C) represents a purely metallic substance. Aluminum oxide consists of Al3+ ions from the metal aluminum and O2– ions from the nonmetal oxygen, so (D) is the correct answer.

16 B A polar molecule must contain polar covalent bonds. Of the choices, only (A), (B), and (E) involve covalent bonding. However, (A) and (E) involve diatomic molecules consisting of one element; we know that these molecules will contain nonpolar covalent bonds, since their atoms won't differ in electronegativity. Carbon monoxide molecules contain different nonmetals, and thus polar covalent bonds. That's why (B) is correct.

105 T, T, CE Divide and conquer! Look at the first statement on its own and decide whether it's true or false. It's true. Look at the second statement by itself. It's true. We know that atoms have different electronegativities.

Let's see if the whole sentence makes sense. “Some covalent bonds are polar in nature because atoms of different electronegativities are unequal in the degree to which they attract electrons.” Does that make sense? What makes a polar covalent bond polar? One of the atoms in the bond hogs the shared electrons; one atom has a higher electronegativity than the other. The sentence makes sense, so fill in oval CE.

106 F, F Divide and conquer. Look at the first statement by itself. Is it true? No! We know that most atoms form bonds because they would like to achieve a stable octet.

What about the second statement? It's false. Ionic and covalent bonding do provide atoms with a stable configuration—;the configuration that resembles a stable octet.

56 A A carbon atom has 4 valence electrons, and an oxygen atom has 6. Using carbon as the central atom and arranging the atoms to give them octets yields Notice that the carbon dioxide molecule consists of 2 double bonds, but no single bonds. The answer is (A).

60 B Determine the structure of SO2. Sulfur is the central atom, and each atom has 6 valence electrons. Arranging them all so that they have octets gives you Notice that there are three electron pair sites around the central sulfur atom (a double or triple bond counts as only one site), one of which is a lone pair. This will result in a bent shape, so (B) is correct.

63 E In the course of this reaction, 4 moles of C–H bonds and 1 mole of Cl–Cl bonds are broken. So 4(410 kJ) + 240 kJ, or 1,880 kJ, are needed to break these bonds. The reaction produces 1 mole of C–Cl bonds, 1 mole of H–Cl bonds, and 3 moles of C–H bonds. So 330 kJ + 430 kJ + 3(410 kJ), or 1,990 kJ, of energy is released by bond making. The net change, which is roughly equal to ∆H, is 1,880 kJ – 1,990 kJ = –110 kJ. Therefore, this reaction is exothermic, and the correct answer is (E).

Chapter 8 Phases: Gases, Liquids, and Solids

10 C Remember that values that are on the same side of the ideal gas equation are inversely proportional. In PV = nRT, both R and T (Kelvin temperature) are on the same side as n (moles of gas). Since R (ideal gas constant) cannot change with a change in moles, the answer must be (C).

11 D In a mixture of gases, each fills the container, so (E) cannot be right. It doesn't make sense to add the gases' temperatures to get a total temperature, so (B) and (C) are eliminated. That leaves us with (A) and (D). The ideal gas constant is not something that could be summed up for each gas either, so the answer is (D). Remember that adding the partial pressures of a mixture gives you the total pressure of the system.

12 C Don't forget that temperature is a measure of average kinetic energy and that for ideal gases, Kelvin temperature increases in proportion to changes in kinetic energy. (C) is correct.

109 T, T It's time to divide and conquer. What do you think of the first statement by itself? It's true; it describes kinetic molecular theory.

The second statement is also true; you know this from the ideal gas law: PV = nRT. Temperature and pressure are on opposite sides of the equation, so they're directly proportional when other variables are constant.

Let's see if the whole sentence makes sense. “If an ideal gas is located in a closed container and temperature is increased, the average speed of the molecules will always increase as well because for an ideal gas, temperature, and moles of gas are inversely proportional.

The sentence does not make sense. Both halves are true, but when they're put together the statement makes no sense. Do not fill in the CE oval.

110 F, T Divide and conquer. The first statement is false. Pressure and volume do have a relationship: PV = nRT.

Now, what about the second statement? It's true. Look at the ideal gas law: PV = nRT. Temperature and volume are on opposite sides of the equation, so they're directly proportional when other variables are constant. The first statement is false, and the second is true.

61 D Don't fall into the temptation trap. In an ideal gas there is no attraction or repulsion between molecules, so eliminate (A) and (B). Gas molecules are in continuous motion in both ideal and nonideal gases, so eliminate (C). In a real gas, molecules will slightly attract each other. As a result, gas molecules strike the container walls with less force. Less force means lower pressure. So pressure is less in the real situation as compared to the ideal. That's why (D) is correct.

62 D This is a partial pressure question. You know that the total pressure of the system is 1,200 torr, and you know that each gas contributes to the total pressure by exerting a partial pressure.

There are a total of 24 moles of gases in the container. The helium concentration is 2 moles/L. We've got 4 L total, so there are 2 × 4 = 8 moles of helium molecules in the mixture. Hydrogen's concentration is 1.5 moles/L, so we've got 1.5 × 4 = 6 moles of hydrogen molecules. 8 + 6 = 14. Since the total number of moles of molecules—;for all 3 gases—;is 24, there must be 10 moles of carbon dioxide molecules in the mixture.

If there are 10 moles of carbon dioxide molecules and 24 moles of molecules total, carbon dioxide's mole fraction = . Total pressure = 1,200 torr, and carbon dioxide's pressure is therefore × 1,200 torr. (D) is correct.

12 E Think back to the first phase change diagram. As heat was removed from the system, the substance moved from gas phase to liquid phase to solid phase. Which process is clearly doing that here? In choice (E), a gas is condensing into a liquid, so (E) is the correct answer.

13 B Remember that sublimation is the direct conversion of a solid into a gas, so (B) is correct.

14 E A decrease in entropy means an increase in order. There is an increase in order (and restriction in molecular motion) as a gas condenses to a liquid. Choice (E) is correct.

15 D Heat of fusion is associated with the process of melting or freezing. Do any choices involve one of these? Yes; in choice (D), solid bromine melts. If this change took place solely at the melting point, then the heat of fusion of bromine would equal the enthalpy change for the process. Choice (D) is the correct answer.

111 T, F You know what to do with this question type by now. Is statement I true or false? It's true. What about statement II? It's false. Hydrogen bonds are stronger than most intermolecular forces but far weaker than covalent or ionic bonds.

112 F, F As you go up into higher elevations, the atmosphere exerts less pressure. As the pressure over a substance decreases, the boiling point of the substance will also decrease. Since water boils at 100°C at sea-level (1 atm), it will boil below 100°C at higher elevations. So statement I is false. Statement II is also false. Average kinetic energy is a measure of temperature, not pressure. So even if the pressure on a sample increases, if its temperature remains constant, so will its average kinetic energy.

52 B Molecules that exhibit hydrogen bonding contain one or more hydrogen atoms (H) bonded to either N, O, or F. The partial positive end of one molecule (the H end) will become attracted to the partial negative end of a nearby molecule (in this case, the N end), creating a hydrogen bond. Choice (B) is correct.

60 C A substance with a low vapor pressure doesn't evaporate readily because it possesses relatively strong intermolecular attractions, so eliminate (A) and (D). What would be the result of strong intermolecular forces? Recall that these must be overcome for a substance to boil. If they are strong, boiling will occur only at relatively high temperatures. Choice (C) is correct.

Chapter 9 Solutions

9 E When ionic solutes dissolve in water, an electrolytic solution is produced. Are there any ionic solutes among the choices? Look for the combination of a metal and a nonmetal. Calcium oxide is ionic, and (E) is the correct answer.

10 A When you are told that solubility increases as temperature decreases, think of dissolving a gas in aqueous solution. Are there any gases in the answer choices? Yes: NO2. The correct answer is (A).

11 E Remember that the degree of boiling point elevation is proportional to the moles of dissolved particles. Choices (A) through (D) are substances composed solely of nonmetals. They are molecular compounds. In general, molecular compounds don't dissociate (acids, which we'll look at later, are an important exception), so 1 mole of the substances in (A) through (D) gives 1 mole of dissolved particles. Calcium oxide is different. As it dissolves, CaO dissociates into Ca2+ and O ions, so 1 mole of CaO yields 2 moles of dissolved particles. CaO is the solute that will most raise water's boiling point; the correct answer is (E).

106 T, T Divide and conquer. Look at the first statement by itself. Is it true? Yes; you should associate “ions dissolved in solution” with the idea of conducting electricity.

The second statement is true. When a solvent can't dissolve any more solute, it's saturated—;the solute has reached the limit of its solubility. Let's see whether the whole sentence makes sense. “Aqueous solutions with ionic solutes conduct electricity because a liquid solvent becomes saturated when the solute reaches the limit of its solubility.” The sentence is nonsense—;do not fill in the CE oval.

107 T, F Again, divide and conquer. Is the first statement true or false? It's tricky but true. For each mole of original solute, a nonionic solute produces only 1 mole of particles in solution. One mole of NaCl, however, dissociates into 2 moles of particles. So the number of particles floating around in the NaCl solution will be twice the number of moles in the nonionic solution. The freezing point depression for the nonionic solution will be one half what it is for the NaCl solution.

What about the second statement? It's false. The freezing point depression constant does vary with the solvent.

27 B You've learned to associate increased temperature with increased solubility of solids in water. This question is simple, and (B) is correct.

34 E The question is about solutions that are capable of conducting electricity. Think of electrolytes, and remember that you should associate them with “ions in solution.” Which of the answer choices lists an ionic compound capable of dissociating in solution? (E) does. Ca is a metal and Cl is a nonmetal, which tells you that CaCl2 is most likely an ionic substance. Choice (E) is the correct answer.

45 A This question asks you to look for the insoluble product of this precipitation reaction. Because precipitations occur through double replacement reactions, the two products formed will be NaCl(aq) and BaSO4(s). If you look back at the solubility rules, you can see that sodium chloride will be soluble; this means that you can eliminate this answer choice, which leaves you with the correct answer, (A).

Chapter 10 Kinetics and Equilibrium

18 E While the surface area of solids and liquids can be increased (through crushing a solid or spraying a liquid), the surface area of gases cannot. Which answer choice has solid or liquid reactants? Choice (E) does, and it's the correct answer.

19 B When the volume of a system is decreased, equilibrium will shift in the direction that produces fewer moles of gas. Eliminate (A) and (E)—;neither reaction involves a gaseous species, so pressure changes will not affect their equilibrium. Now, look at the other three choices: Which reaction involves fewer moles of gas on the right than on the left? Choice (B) does (3 moles on the left, and 2 on the right). So the correct answer is (B).

20 A We're asked about reverse reactions, so read choices (A) to (E) from right to left. Recall that only gaseous and aqueous species can have their concentration increased. Choices (B), (C), (D), and (E) have either a gaseous or aqueous species on the right side (this is a bit tricky—;these would be the reactants of the reverse reaction). That leaves A, which has a single solid reactant for its reverse reaction. The concentration of pure solids cannot be changed, so (A) is correct.

109 F, T Divide and conquer! Assess the first statement by itself. Is it true or false? If you increase the concentration of a product, equilibrium shifts in the direction that creates more reactant, so in this case, to the left. This means that we will see an increase, not a decrease, in the concentration of reactants. The statement is false.

What about the second statement? It is a statement of Le Chatelier's principle, so it's true. The first statement is false, and the second is true.

110 F, F Divide and conquer. Is the first statement true or false? Don't be tricked! At equilibrium, the rates of forward and reverse reactions are equal, but the concentrations of products and reactants are usually not. This statement is false.

Let's look at the second statement. What do they mean by the “right side of any equilibrium expression”? Equilibrium expressions, remember, look like this:

Keq = What's on the right side? The fraction showing product concentrations and reactant concentrations. As we know, those are not usually equal at equilibrium, so both statements are false.

111 T, T As concentrations of reactants increase, so do molecular collisions, and this produces a higher reaction rate. Statement I is true.

What about statement II? It's also true. As collisions become more energetic, they are more likely to lead to product formation.

Now put the sentences together. Does it make sense? It sounds good, but don't fall into the temptation trap: Only a temperature change, not a concentration change, can change the energy of molecular collisions. Don't fill in the CE oval.

53 B Remember what we said about the size of Keq:

 An equilibrium constant > 1 a reaction that favors the forward direction An equilibrium constant < 1 a reaction that favors the reverse direction

The Keq is less than 1 for the first reaction and greater than 1 for the second, which means that the reverse direction is favored in the first reaction, and the forward direction is favored in the second reaction. Choice (B) is correct.

56 C Think of Le Chatelier's principle, and look at what's happening here. Someone increases the concentration of the product O2 in a reaction at equilibrium. This means that the concentration of the other product, SO2, will decrease, and the concentration of the reactant(s) will increase. Choice (C) is the correct answer.

58 B Don't fall into the temptation trap. You've probably heard the phrase “not consumed by the reaction” in connection with catalysts, in kinetics. But here they're asking about an activated complex. An activated complex is quickly broken down to form the products of a reaction. Choice (B) is the correct answer.

Chapter 11 Acids and Bases

17 B Litmus is an indicator that is blue in basic solutions. Since NH3 is the most obvious base among the choices, the answer is (B).

18 B At 25°C, a pH greater than 7 indicates a basic solution. (B) is correct.

19 C A nonelectrolyte does not dissociate into ions in water. Soluble ionic compounds, strong acids, and strong bases are all strong electrolytes, so eliminate (A) (a strong acid). Weak acids and bases ionize to a slight extent, and, therefore, are weak electrolytes. So eliminate (B) (weak base) and (D) and (E) (both weak acids). What's left? Water. But doesn't water ionize to a slight extent? Yes, but check out how slight: Kw (equilibrium constant for the ionization of water) at 25°C is 1.0 × 10–14. That's so small that we can consider water to be a nonelectrolyte. (C) is correct.

20 A The same species that makes strong electrolytes (soluble ionic compounds, strong acids, and strong bases) has an ionization reaction that essentially goes to completion. Are any of these answer choices strong acids or bases? Yes. HBr is a strong acid. It completely ionizes into H+and Br ions in water, so (A) is correct.

110 F, T Divide and conquer! Look at statement I by itself, and decide if it's true or false. It's false. If you add an acid to neutral water you increase the hydrogen ion concentration and decrease the hydroxide ion concentration.

Is the second statement true or false? This is a true statement about water's ion product. So, statement I is false, and statement II is true.

111 F, F Remember the six common strong acids? HI is one of them, so statement I is false.

What about the second statement? Because HI is a strong acid it will donate, not accept, H+ ions. Both statements are false.

50 C Conjugate acids and bases appear on the right side of the equation, so eliminate (A) and (B). A conjugate acid donates an H+ ion. Does either H2O or NO3 do that? Yes: H2O donates an H+ ion to NO3 to re-form the reactants HNO3 and OH. So H2O is the conjugate acid, and the answer is (C).

53 A We know that if we fully titrate an acid we need equal amounts of H+ and OH ions. There are 2 moles of OH ion for each mole of Ba(OH)2, and we're dealing with 0.015 L of a 0.015 M solution, so

(2)(0.015)(0.015) = 0.00045 moles OH

We need 0.00045 moles of H+ ion, too. We have 0.03 L of the HCl solution, and there's just 1 mole of H+ ion per mole of HCl, so

(1)(0.03)(x) = 0.00045 moles OH
x = 0.015 M

Choice (A) is the correct answer.

55 D H3PO4 looks like the formula of an acid, and it is—;phosphoric acid. Choices (B), (C), and (E) are properties of bases, so eliminate them. Is H3PO4 on our list of strong acids? No; so it must be a weak acid. Would a weak acid have a large Ka? No; so eliminate (A). A weak acid is a weak electrolyte. Choice (D) is the correct answer.

Chapter 12 Redox and Electrochemistry

11 C Remember the parts of an electrochemical cell and what they do. A salt bridge neutralizes charge buildup in the half-reaction vessels. Choice (C) is the answer.

12 A Did you remember AN OX and RED CAT? If so, then you know that oxidation occurs at the anode. The correct answer is (A).

13 E You might have been tempted to pick (C). However, a salt bridge doesn't physically connect the anode to the cathode. An electrical wire (whether or not a voltmeter is also attached) connects the electrodes, so (E) is correct.

14 B Think about what happens during the operation of an electrochemical cell. The anode is oxidized into an aqueous cation. That causes the anode to decrease in mass as the redox reaction progresses. The opposite happens at the cathode. An aqueous ion acquires electrons to form more of the metal that composes the cathode, so the cathode gets more massive as the reaction happens. The answer is (B).

104 T, T, CE Divide and conquer. Look at the first statement, and evaluate it on its own. Is it true? Yes. Wherever there's oxidation, there must be reduction.

Look at the second statement by itself. Is it true? Yes. If one species gives up electrons, something else must accept them. Now see whether the whole sentence makes sense: “Any reaction in which one atom is oxidized requires that another atom be reduced because if one species donates electrons, another must acquire them.” Does it? Absolutely. Oxidation means losing electrons, and reduction means gaining electrons. Oxidation must accompany reduction because if electrons are lost from one place, they must be gained by another. The second statement explains the first, so fill in oval CE.

105 T, F Evaluate the first statement on its own: Does electrolysis require the input of electricity? It sure does, so statement I is true.

What about the second statement? A spontaneous redox reaction has positive overall E0, so statement II is false.

32 B Do some simple oxidation/reduction arithmetic. Sodium starts with an oxidation state of 0 because it isn't in a compound. When Cl is in a compound, its oxidation state is usually –1. In NaCl, Cl has an oxidation state of –1, and Na has an oxidation state of +1. The oxidation state of Na was 0 and is now +1; Na has lost electrons—;it has been oxidized. Choice (B) is the correct answer.

35 A Again, some simple oxidation/reduction arithmetic is necessary. Oxygen's oxidation state here is –2. Since there are 4 oxygen atoms in the formula, oxygen contributes total oxidation of (–2)(4) = –8. Potassium's oxidation state is +1. The oxidation state of the overall compound is 0. So 1 + (–8) + x = 0. x = +7. That's why (A) is correct.

54 E Let's begin by applying the rules for assigning oxidation states to the given reaction.

2Al0 + 6H+1 Cl–1 → 2 Al+3Cl3–1 + 3H20

Notice that each mole of Al that's oxidized to Al+3 loses 3 moles of electrons, so 2 moles of Al lose 6 moles of electrons during oxidation. The 6 moles of electrons that are given up by Al are acquired by H+ ions to form H2. Choice (E) is the correct answer.

Chapter 14 Laboratory

1 C According to the chart on this page, barium salts will burn with a green color. We have no information about the burning characteristics of (A), (B), or (D).

2 A Due to its relative volatility, ethyl alcohol will evaporate at a lower temperature than water. This is a common procedure for the concentration of this substance. Although phenolphthalein (B) may also be separated by distillation, this is not a common procedure.

3 E A U-Tube manometer will show a rising mercury level as a heated gas pushes with a greater force per unit area (pressure).

4 D Of our common indicators, only litmus is red in acid and blue in basic solution. Phenolphthalein is clear in acid, and pink in basic solution.

105 T, F The indicator will change colors as soon as the solution becomes slightly basic, which will occur after equivalence, so statement I is true.

This is a deceptive statement because, although a neutral solution will have an equal molar amount of H+ and OH ions, the actual volumes of solution required to achieve this depend on the concentrations, so statement II is false.

106 F, T Hexane (C6H8) has a higher molar mass, and is of the same class of molecule as pentane (C5H7) and therefore will have a higher boiling point, so statement I is false.

Vapor pressure is a measure of a substance's propensity to vaporize and due to its lower mass, pentane's will be higher than hexane's, so statement II is true.

7 B To find mass in grams, we need only multiply the isotope mass by the number of moles present. 2.5 mol × 2.014 g/mol = 5.035 g. Only (A) reflects both the proper calculation AND the right number of significant figures.

8 B When NaOH and HCl neutralize, water and NaCl salt are produced. After evaporation, the solid will burn yellow (B) in a flame test. (C) and (D) are the colors for different metallic salts, while (A) and (E) are very unlikely in a standard flame test procedure.

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