Cracking the SAT Chemistry Subject Test

Part IV

Practice SAT Chemistry Subject
Tests and Answers and Explanations

Chapter 16

Practice SAT Chemistry Subject Test 1: Answers and Explanations

PART A

1 A When we talk about specific heat, we're talking about the amount of heat necessary to produce a change in temperature. The calorimeter can be used to measure heat input or output, and the thermometer, choice (A), would be used in combination with it to ascertain the associated change in temperature.

2 E A graduated cylinder can be used to help find the volume of an irregularly shaped solid. How? Fill the graduated cylinder with water, and read the water's volume. Next, add the solid. The difference between the volume of both the water and solid and the volume of the water alone is the volume of the solid.

3 D When an acid and base are combined, think titration, and when you consider titration remember that a buret is typically used to deliver small amounts of acid into a base and vice versa.

4 C Electrons travel from the anode to the cathode in an electrochemical cell. But what allows the redox reaction to go on by maintaining charge neutrality in each vessel? That's the function of a salt bridge. The salt bridge completes the circuit of the electrochemical cell.

5 B An amphoteric molecule can act either as an acid or a base. Proteins are polypeptides made from amino acids, and all amino acids have both an acid group (carboxylic acid group) and a base group (amino group). Therefore, proteins are always amphoteric.

6 C Proteins and carbohydrates are both polymers; however, only carbohydrates commonly form branched polymers. Glycogen and cellulose are both carbohydrate polymers made up of glucose monomers; glycogen is a highly branched polymer while cellulose is primarily straight-chained.

7 C Deoxyribose is a ribose sugar molecule missing an oxygen atom. As with all molecules with the suffix -ose, ribose is a carbohydrate. In general, proteins tend to have the suffix -in (or -ase if they are an enzyme) and nucleic acids have the suffix -ine (except for uracil).

8 E Electrolytes are substances that increase the electrical conductivity of water by dissolving in solution to form ions. Therefore, all ionic compounds, or salts, are electrolytes.

9 D Most lipids are insoluble in water. For example, fat-based oils (such as corn oil), a subfamily of lipids, form droplets in water. Several other fat derivatives form double-layered sheets in water; this type of lipids serves as the principle structural element in cell membranes.

10 E Don't let phrases such as “constituent elements” throw you off course. The question asks you to identify a situation in which a compound is broken down into its elements. Mercury(II) oxide, HgO, is decomposed into the elements mercury, Hg, and oxygen, O2, in choice (E).

11 C When a radioactive atom undergoes alpha decay, it loses 2 protons and 2 neutrons. That means that its atomic number decreases by 2, and its mass number decreases by 4. That's exactly what has happened here. Uranium (atomic number = 92, mass number = 234) has been converted to thorium (atomic number = 90, mass number = 230).

12 E The phrase “oxidation-reduction reaction” describes a reaction in which one atom loses electron(s) to another. The atom that loses electrons is oxidized, and the one that gains electrons is reduced. In HgO, the oxidation state of Hg is +2 and that of oxygen is –2. HgO is decomposed into the free elements Hg and O2, each of which has an oxidation state of 0. So the oxidation state of Hg goes from +2 to 0; it has been reduced. Oxygen has been oxidized; its oxidation state has changed from –2 to 0. This is clearly a redox reaction.

13 B Look at choice (B). In an atom of carbon-14 there are 8 neutrons and 6 protons, a ratio greater than 1. In nitrogen-14, the neutron-to-proton ratio is 7:7 or equivalent to 1. Choice (B) is an example of beta decay. As you can see, beta decay causes the neutron-to-proton ratio to decrease.

14 E The activation energy of forward and reverse reactions is always characterized by the “hump” that you see in pictures of this kind. It's the energy necessary to get the reaction going. The reactants of the reverse reaction have energy that is expressed by the flat portion of the curve to the right of the hump. For a reaction to occur, these reactants must gain an energy equal to that represented by the top of the hump. This energy that must be acquired is represented by (E). Remember that catalysts reduce activation energy and the rate of the reaction.

15 B The enthalpy change of a reaction is the amount of heat the reaction absorbs or gives off. In this case, the reactants begin at one energy level (represented by the flat portion of the curve to the left of the hump), and the products are associated with another (represented by the flat portion of the curve to the right of the hump). The difference represents the enthalpy change of the reaction (which, in this case, is negative—;the reaction liberates heat; it's exothermic).

16 A Recall that the activated complex represents the highest energy state reactants achieve as they are transformed into new substances. So the energy of the activated complex is measured from the very bottom of the diagram to the top of the activation energy barrier. This distance is represented by (A).

17 B BaI2 is composed of a metal (Ba) and nonmetal (I) bonded together. This is an ionic compound that held together by—;surprise—;ionic bonding.

18 C You may be tempted to go with “ionic bonding” here, but resist that impulse. The ions in an ionic solid are too restricted in their movement to conduct a charge, so (B) is incorrect. Now think: What solids conduct electricity? Metals, of course. And why can copper wire be used to conduct electricity? Because the metallic bonds that hold a sample of copper together do so through the motion of many free electrons, which can conduct electricity as they move.

19 D Don't be fooled by (A). Hydrogen bonds occur between, not within molecules. A hydrogen molecule consists of nonmetal hydrogen atoms in a bond. Nonmetals form covalent bonds, and identical nonmetal atoms form nonpolar covalent bonds.

20 A Water's vapor pressure (its tendency to evaporate) is low compared to other similarly sized molecules. What keeps molecules together in the liquid state? Intermolecular forces do, and the intermolecular force most prevalent in water is hydrogen bonding. Since hydrogen bonds are a relatively strong intermolecular force, water molecules are significantly attracted to each other, and water does not evaporate readily.

21 B You might be asking yourself: purplish vapor? How am I supposed to know that? Unfortunately there will be a few questions on the test that will test your familiarity with the properties of certain substances. We hope your experiences in chemistry lab will carry you through. If not, don't panic. You'll see only a few of these types of questions. Iodine is a grayish-purple solid that gives off a similarly colored vapor as it sublimes.

22 E If you don't know that graphite, a form of carbon, can conduct electricity, you can still get the answer by eliminating the other choices. Choices (A) and (C) are ionic solids—;they can conduct electricity in solution or in the molten state, but not as solids. Choices (B) and (D) (table sugar) are molecular solids. You wouldn't expect molecular solids to be particularly conductive. That leaves graphite, which is a network solid.

23 C If you've dissolved sodium hydroxide pellets in a beaker of water and felt the side of the beaker, you know that the process gives off heat.

PART B

101 T,T Use the divide and conquer strategy. Carbon is a nonmetal, so statement I is true. Do carbon atoms bond with each other? They sure do. Otherwise we wouldn't have oils, waxes, fossil fuels, diamonds, and literally thousands of different substances. Now, does the sentence make sense? No. Metal atoms can also bond with each other, so this ability is not unique to nonmetals. Fill in both true ovals, but not the CE oval.

102 F,T Isotopes of the same element do not have the same mass number, but since they are the same element, their atomic numbers are identical. The first statement is false and the second is true.

103 F, F Divide and conquer. At a given temperature, the density of water stays the same whether we have 10 grams or 20 grams, so statement I is false. Statement II is also false: Molecules in the liquid (and solid) state are much closer than they are in the gaseous state.

104 T,T,CE Both sodium and cesium are in the alkali metal family. As such, they have similar chemical properties, so statement I is true. Statement II is also true: Alkali metals such as sodium and cesium have 1 valence electron in their atoms. Do the two statements make sense when they are combined? Do sodium and cesium exhibit similar chemical properties because their atoms have the same number of valence electrons? Yes, so fill in oval CE.

105 T, T, CE Divide and conquer. Can an endothermic reaction be spontaneous? Have you ever seen an ice cube melt at room temperature? That's a spontaneous endothermic process, so the first statement is true. What about the second statement? Remember that the change in Gibbs free energy, ∆G, depends on enthalpy change, ∆H, and entropy change, ∆ S: ∆G = ∆HTS. Statement II is also true. Does the second statement explain the first? Yes, it does. That ice cube melts at room temperature because the increase in entropy for the process overcomes the change to a higher energy state. Fill in the CE oval.

106 T, T, CE The first statement is true. The 3d orbitals are of higher energy than the 4s orbital, so the 4s orbital fills first. Evaluate the second statement. It is true. Subshells do fill in order of lower to higher energy. Does the second statement explain the first? Yes, it does. The 4s orbital fills before the 3d orbitals because it is lower in energy. Fill in oval CE.

107 T, T Divide and conquer. Here's what happens when calcium and bromine react: Ca + Br2 → CaBr2. Bromine's oxidation state decreases from 0 to –1; it is reduced. Calcium (which is oxidized) is responsible for reducing bromine. In other words, calcium acts as a reducing agent. Statement I is true. Look at statement II. Is mass conserved in a chemical reaction? Yes. If it weren't, there would be no need to balance equations. Both statements are true. Does statement II explain why statement I is true? No, it doesn't. Do not fill in oval CE.

108 F, T Adding an acid to water increases the hydrogen ion concentration in the water, which means that the water's pH is reduced. The first statement is false and the second is true.

109 F, F For a liquid to boil, the intermolecular forces in the liquid must be overcome, not the bonds within individual molecules. When water boils, its H2O molecules are still intact. While we're considering boiling, take a look at statement II. You need to heat water to make it boil, so boiling absorbs, not releases, heat. Both statements are false.

110 T, F Divide and conquer. A Geiger counter is used to detect radioactive particles, so statement I is true. Be careful with statement II. Radioactive elements have atoms with unstable nuclei. However, that has nothing to do with an atom's valence electrons. Radon (Rn) is a perfect example. The nuclei of radon atoms emit alpha particles. However, radon is a noble gas. Radon atoms have filled valence shells and are therefore unreactive chemically. Statement II is false.

111 T, T, CE The first statement is true. If the temperature of a substance didn't remain constant during melting there would be no such thing as a melting point. Instead, at a given pressure, a substance would melt over a range of temperatures. Statement II is also true. The heat absorbed by the ice is being used to break intermolecular hydrogen bonds, so we don't see the temperature rise although heat is being added. Does the second statement explain the first? Yes, it does. Since the average kinetic energy of molecules stays constant during a phase change (such as melting), the temperature also remains constant. Fill in the CE oval.

112 T, F The first statement is true. Adding a solute to a solvent reduces its freezing point, raises its boiling point, and reduces its vapor pressure. The second statement is false. Some, but not all, solutes dissociate into positive and negative ions.

113 T, T Divide and conquer. Statement I is true. A reaction will proceed more quickly if its temperature is raised. Look at the second statement. A large Keq absolutely means that a reaction favors the forward reaction or, in other words, favors product formation. Both statements are true. Put them together. Does the second explain the first? No, it doesn't. The first deals with reaction rates (kinetics) and the second with equilibrium. These are different areas of chemistry, so don't fill in CE.

114 T, F Hydrofluoric acid is not one of the six common strong acids; thus, it will partially ionize and is a weak electrolyte. Hydrochloric acid is a strong acid, and it ionizes completely. So HCl(aq) is a strong electrolyte. Statement I is true. The second statement is false. Remember that electronegativity values decrease down a given column. So from fluorine to chlorine, electronegativity decreases.

115 T, T, CE The first statement is true. An example of this is the carbon tetrachloride molecule, CCl4. It consists of four polar bonds. However, the bonds are arranged such that the overall molecule is nonpolar. Therefore, the second statement is true. Since the second statement explains the first, fill in the CE oval.

116 F, T Divide and conquer. Does electrolysis generate electricity? No, electrolysis involves the use of electrical energy to force a chemical reaction to occur, so statement I is false. What about statement II? It's true. Electrons flow from the anode to the cathode in both electrochemical and electrolytic cells.

PART C

24 D The atomic number is the number of protons in the nucleus, and the mass number is the sum: number of protons + number of neutrons. If the atomic number is 39 and the mass number is 89, then the number of neutrons in the nucleus must be (89) – (39) = 50.

25 C Use the plug-in balancing strategy. Since there are at least 4 carbon atoms on the left, the coefficient of CO2 cannot be 2, so eliminate (A). If the coefficient of carbon is 4, we must place a 1 in front of C4H10 to keep carbons in balance. This will give 10 hydrogens on the left. If we put a 5 in front of H2O on the right, we then have 13 oxygens on the right. The only way we can get 13 oxygens on the left is to place in front of O2 on the left. This puts all the elements in balance, but violates the rule of using only whole numbers, so (B) is wrong. However, if we multiply the coefficients we just determined by 2, we will maintain balance and have all whole numbers. So the balanced equation becomes: 2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(l).

26 C The mass of a proton is approximately 1 amu, and this is very nearly the mass of a neutron. Both a positron and an electron are much lighter than 1 amu. A hydrogen molecule weighs roughly twice as much as a proton, and an alpha particle weighs about four times as much.

27 D Add up the mass of 1 mole of this substance. From the periodic table, we know that

• 1 mole of hydrogen atoms has a mass of about 1 g.

• 1 mole of chlorine atoms has a mass of about 35 g.

• 4 moles of oxygen atoms have a mass of about 64 g.

The total gram-molecular weight of this substance, then, is 100 g. Oxygen's contribution is 64 g, which means the compound is 64 percent oxygen by mass.

28 E As soon as you hear the term electron sharing, you know you're dealing with a covalent bond, so eliminate (A), (B), and (C). The fact that the two atoms differ in electronegativity tells you that one has more attraction for the shared electrons than the other. The result? A polar covalent bond—;the molecule has a negative and a positive pole.

29 B Use q = mcT to compute the amount of heat transfer. For water, the specific heat, c, is about 1 calorie/g • °C, so a 20 g sample of water experiencing a 20°C increase in temperature has (20 g)(1 calorie/g • °C)(20° C), or 400 calories of heat, transferred to it.

30 D Remember the oxidation state rules. An oxygen atom usually has a –2 state. Potassium atoms are always given a +1 state. In K2Cr2O7, we have 2 potassium atoms and 7 oxygen atoms. So potassium atoms contribute 2(+1), or a state of +2. Oxygen atoms contribute 2(–7), or –14. For K2Cr2O7 to be neutral, each chromium atom must have a state of +6.

31 B If the pH is 5, then [H+] is 1 10–5 moles/L. Water's ion product is 1 10–14 at 25°C, meaning that the product [H+] [OH] is 1 10–14. So [OH] = (1 10–14 )(1 10–5) = 1 10–9 moles/L. Choice (B) is correct.

32 D Based on the balanced equation, the ratio of water vapor consumed to oxygen produced is 2 moles H2O to 1 mole O2. The volume of gas will also be in this 2:1 ratio. So 89.6 liters of H2O(g) are required to produce 44.8 liters of O2(g).

33 B Molarity refers to moles of solute per liter of solution. We know we have 500 milliliters of solution, but we don't know how many moles of solute we have. Let's first figure out the mass of 1 mole of MgCl2. Looking at the periodic table, we find that 1 mole of Mg has a mass of 24.3 g. Two moles of Cl have a mass of about 71 g. One mole of MgCl2, therefore, has a mass of 95.3 g.

Now, we're dealing with 190 g of MgCl2, which is equal to = about 2 moles. But don't be duped into choosing (A)! You're looking for the solution's molarity. There are 2 moles of solute in 500 milliliters (0.5 liters) of solution, which means that the molarity is 4 moles/L. That's why (B) is correct.

34 C Think of the ideal gas equation: PV = nRT. What does this equation tell us? It means that volume is directly related to Kelvin temperature and inversely related to pressure. Doubling a gas's Kelvin temperature will double its volume if other variables are held constant. Doubling a gas's pressure will halve its volume if other variables are held constant. So the effect on volume of doubling pressure cancels out the effect of doubling Kelvin temperature, and the net result is that the gas's volume will stay the same.

35 C After 1 half-life, a 100 g sample will have a mass of 50 g. After 2 half-lives, it will have a mass of 25 g, so we're talking about the expiration of 2 half-lives. If 2 half-lives = 12.4 hours, then 1 half-life = 6.2 hours.

36 A This is alpha decay, in which a radioactive atom loses 2 protons and 2 neutrons. The loss of 2 protons means the atomic number decreases by 2, which means it was 87 before it decayed. The element with an atomic number of 87 is Fr (francium). The loss of 2 neutrons together with the loss of 2 protons means that the mass number has decreased by (2) + (2) = 4. The mass number is now 216, which tells us that it was 220. Notice that choice (C) has the right numbers, but not the right element. Remember that the atomic number uniquely identifies an element, so an atomic number of 87 must be francium, no matter what the mass number is. This also means that an element can have several different mass numbers, collectively called isotopes.

37 C The empirical formula tells us that the ratio of carbon to hydrogen is 1:2, so we're looking for an answer that reflects the same ratio. Only choices (B), (C), and (E) do that, so we can eliminate (A) and (D). Now, among (B), (C), and (E), we're looking for the one whose molecular weight is 56 amu. Look at the periodic table. Every carbon atom has an atomic weight of 12 amu, and every hydrogen atom has an atomic weight of 1 amu. Rather than pursue algebra, let's just try the three choices.

38 A Don't fall into the temptation trap and pick (B). Temperature differences can indicate the direction of heat flow. However, temperature is not a direct measure of heat energy. Instead, associate heat energy with enthalpy.

39 E On this test, the easiest way to balance an equation is by plugging in the answers. The test writer tells you that the coefficient in front of H2O is 6, so put that in there. Now, see which of the answer choices, if placed in front of NF3, would result in a balanced equation. If, for instance, we try choice (B), we'd have 1 mole of N on the left, which would give us 3 moles of F on the left. In order to have 3 moles of F on the right we'd have to put a 3 in front of the HF on the right. That means we'd have 3 moles of H on the right and 12 moles of H on the left. This is way out of balance.

Suppose we try option (D) and put a 3 in front of the NF3 on the left. That gives us 9 moles of F on the left, which means we'd have to put a 9 in front of the HF on the right. That in turn would provide 9 moles of H on the right when we have 12 moles of H on the left. Once again, this is out of balance.

Now, let's try choice (E). We put a 4 in front of NF3 on the left, which gives us 12 F on the left and means we must put a 12 in front of the HF on the right. That gives us 12 H on the right, balanced by 12 H on the left. The coefficients for both NO and NO2 would be 2, which would balance both the nitrogens (N) and the oxygens (O).

40 B Make sure you can distinguish between an acid and a base. A base can donate an unshared electron pair, according to the Lewis definition, so (B) is correct. Choices (A), (D), and (E) are characteristics of acids. And what about (C)? Not all bases contain the OH ion in their structure—;NH3 is an example.

41 C The substance described has some metallic characteristics (shiny and high melting point) and some nonmetallic ones (brittle and poor electrical conductivity). It sounds like something that's between a metal and a nonmetal such as a metalloid or semimetal. This is a general description of the metalloid silicon.

42 C The more free that molecules are to move around, the greater the entropy of the state. In equation I we go from a gas (very high entropy) to a liquid (more ordered, less entropy). That's a decrease in entropy. Equation II also involves an entropy decrease. Here, ions go from being able to move throughout a solution to being restricted in the solid state. Equation III shows an entropy increase because we are increasing the moles of gas (2 on left and 3 on right). I and II illustrate an entropy decrease, and the correct answer is (C).

43 E Choices (A) and (D) will not change water's boiling point. Remember that when a solute is dissolved in solution, the solution's boiling point will be raised (and its freezing point lowered). Only (E) involves dissolving a solute into water. Sugar water boils at a higher temperature than pure water under identical conditions. Why doesn't the addition of gasoline into the water have the same effect? Gasoline molecules are nonpolar and will not dissolve in water. As for (C), increasing the altitude of the water will decrease its boiling point.

44 D The question concerns periodic table trends and, in particular, atomic radius. As we move from left to right across a period, atomic radius decreases. So, within a period, the higher the atomic number, the smaller the atomic radius.

This question is just a bit spiced with the camouflage trap. You might be thinking higher atomic number, and the correct answer is phrased as “greater positive charge in its nucleus.” But keep the blinders off your brain. You know the answer—;just remember that there's more than one way of expressing it.

45 C Aluminum is a metal, and oxygen is a nonmetal. They react to form an ionic compound. Aluminum (in the 3A group) forms a +3 ion. Oxygen (in the 6A group) forms a –2 ion. They will produce aluminum oxide, Al2O3. When the equation is balanced, we'll get 4Al(s) + 3O2(g) → 2Al2O3(s).

46 E Many colored compounds contain a transition metal (an element from the d region of the periodic table). Choices (A), (B), and (C) are ionic solids that possess an active metal (an element from the s region). These compounds appear white (for instance, NaCl or table salt). Choice (D) is rust, which is not blue. CuSO4 contains the transition metal copper (Cu), and its crystals are bright blue.

47 D Remember: The atomic weight of an element is the weighted average of all the different isotopes an element exists in. If 50 percent of element X had a mass of 210 amu and 50 percent had a mass of 214 amu, the weighted average would be 212 amu. Notice that we are told element X exists as 214X more than half of the time. So the answer must exceed 212. However, since element X also exists in an isotope with a mass less than 214, we expect that its atomic weight is less than 214 amu. Only D has a mass greater than 212 amu and less than 214 amu.

48 B The container holds a total of 6 moles of gas. Oxygen (O2) constitutes one-third of that content. If you know how to work partial pressure problems, you know that oxygen's contribution to the total 760 torr of pressure is one-third. 760 torr / 3 = approximately 253 torr.

49 D Since we're considering the dissolution of an ionic solid into water, we need to consider the solubility product expression. The solubility product constant, Ksp, will equal the product of aqueous ion concentrations raised to their coefficients. (Remember that solids are not expressed.) This is the relationship expressed in choice (D).

50 D The normal electron configuration for magnesium would be 1s2s22p63s2. Since we're talking about an “excited” magnesium atom in which one electron has been pushed up into a higher energy state, we're looking for a configuration that shows one electron in a higher state than it should be. The total number of electrons should still be equal to magnesium's atomic number, but the location of one electron should be “elevated.”

Choice (D) is just what we're looking for. Total number of electrons? Twelve, just as it should be. But look at the last entry. Instead of 3s2, we see 3p1. The last electron has been elevated to the 3p subshell.

51 A To form an electrolytic solution, the solute must dissociate into ions. Adding HCl(g) to water will produce hydrochloric acid, which ionizes into H+ and Cl ions. Choices (C), (D), and (E) are all ionic solids, which will break into mobile ions upon dissolution in water. When N2(g) is dissolved into water, no ions are produced, and the resultant solution is nonelectrolytic.

52 C The double arrow indicates that the reaction is reversible. NH4+ is a reactant of the reverse reaction; if NH4+ donates a proton to HCO3, NH3 and H2CO3 are formed. Since NH4+ donates an H+ ion (or proton) to another substance, it acts as an acid according to the Bronsted-Lowry definition.

53 A Add the superscripts to get the total number of electrons in the species: 2 + 2 + 6 + 2 + 6 = 18. Which of the choices also has 18 electrons? A quick check of the periodic table shows that a sulfur atom has 16 electrons. Adding two more electrons gives the S2– ion a total of 18 electrons.

54 B For a substance to be amphoteric, it must be able to donate and receive an H+ ion. Eliminate (A) and (E)—;these species don't have an H+ ion to donate. Choice (C), KOH, is a strong base. We wouldn't expect it to ever act as an acid. Likewise, HNO3 (choice (D)) is a strong acid that we would not expect to behave as a base. That leaves HSO4. Notice that it can act as an acid and become a sulfate ion, SO42–, or act as a base and become sulfuric acid, H2SO4.

55 E The ideal gas equation is PV = nRT. When the amount of gas does not change, n becomes a constant, like R. A little algebra gives us = nR. Since is equal to a constant, it will not change with time. Consider that we are dealing with two points in time. At first, the gas has a volume of 10 liters at 20°C or (293 K) and 750 mmHg. So here, = . Later, the gas is under STP conditions, so T = 273 K and P = 760 mmHg. Now, = . If we set initial = final , we get . Rearranging and applying some algebra gives us V = . This is choice (E).

56 A Use the phase diagram. At 0.5 atm and 200 K, substance Z is a gas. If we maintain this temperature and increase the pressure, we can draw a vertical line from the point (0.5 atm, 200 K). Eventually that vertical line will cross into the liquid region. This means that, under steadily increasing pressure, substance Z (starting at 0.5 atm and 200 K) will condense. Condensation is the phase change from gas to liquid.

57 C The normal boiling point is the temperature at which the phase change from liquid to gas occurs, at a pressure of 1 atm. If you extend a horizontal line from the 1.0 atm mark on the “pressure” axis and see where it intersects the liquid-gas boundary, you'll get the normal boiling point. Doing so on this phase diagram shows a normal boiling point of about 300 K.

58 B Phosphorus is the central atom in PCl3. A phosphorus atom needs 3 electrons to complete its valence shell. It gets 3 electrons by forming covalent bonds with 3 chlorine atoms. The PCl3 molecule has the following structure:

Of the four electron pair sites around phosphorus, one is a lone pair. This gives the PCl3 molecule a trigonal pyramidal shape.

59 A First consider the neutralization that occurs between HBr and Ba(OH)2.

2Br + Ba(OH)2 → BaBr2 + 2H2O

Notice that for every 2 moles of HBr, only 1 mole of Ba(OH)2 is needed for neutralization. We have 0.1 liters (or 100 milliliters) of 0.2 M HBr. This means we have 0.1 liters × 0.2 mole/liter, or 0.02 mole of HBr. We need 0.01 mole of Ba(OH)2 to neutralize 0.02 mole of HBr. Twenty-five milliliters of 0.4 M Ba(OH)2(aq) has 0.025 liters × 0.4 mole/liter, or 0.01 mole of Ba(OH)2.

60 A A small Ka indicates a weak acid. That means statement III is false, and therefore we can eliminate choices (D) and (E). It also means that most HCN remains as intact molecules, as opposed to H+ and CN ions, so statement II is false. Process of elimination tells us that statement I must be true. And it is: HCN H+ + CN. Notice that the molar ratio of H+ to CN is 1:1.

61 E Ionization energies get very large once we try to remove core electrons, which are attracted more strongly to the nucleus than valence electrons. So an atom with a very high second ionization energy would be expected to have just 1 valence electron: The second electron to be removed from such an atom would have to be a core electron. Among the choices, only sodium atoms have a single valence electron.

62 B Fe2+ is oxidized in the redox reaction. To get the standard oxidation potential for Fe2+ → Fe3+ + e, just take the opposite of the standard reduction potential. So E0ox for Fe2+ → Fe3+ + e is –0.77 volts. Cl2 is reduced for the reduction half-reaction, and E0red is 1.36 volts. The potential difference for the overall reaction (E0cell) is –0.77 volts + 1.36 volts = 0.59 volts.

63 D Remember that many metals react with acids to produce hydrogen gas. Your first step should be to write out the reaction. Here it is: Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g). Notice that the products of this reaction are Zn2+(aq), Cl(aq) (from ZnCl2), and H2(g).

64 E This is a Le Chatelier's principle question. How can we produce crowding on the left side of the equation and drive equilibrium to the right (increasing the SO2 concentration)? Don't be fooled by (C): Catalysts do not affect equilibrium, but changing concentrations does influence equilibrium. Increasing the concentration of O2 will produce crowding on the left side and lead to an increase in the concentration of SO2.

65 A Decreasing the volume of the system will increase the concentration of reactants. Why? Because the same number of molecules now exists in a smaller space, and this increases the ratio of molecules per volume. How will this affect the reaction rate? It will increase reaction rate because decreasing the volume makes it more likely that molecules will collide. What about equilibrium? Reducing the volume will force the equilibrium to shift in the direction that produces fewer moles of gas. This means the equilibrium concentration of reactants will decrease because equilibrium will shift to the right (4 moles of gas on the right versus 5 on the left). Only a temperature change will affect the value of Keq, so this will stay the same. Among the three, only item I will increase, so the answer is (A).

66 B When quantities are given for more than one reactant, you must see which is limiting. Fifty-six grams of CO (molecular weight = 28 amu) are 2 moles of CO. Since the stoichiometric ratio of Fe2O3 to CO is 1:3 (based on coefficients from the balanced equation), we see that CO is limiting. (We would need more than 9 moles of CO based on the 1:3 ratio for Fe2O3 to be limiting in this case.) The ratio of CO to Fe is 3:2. So 2 moles of CO will produce about 1.3 moles of Fe. You could quickly estimate this as between 1 and 2 to save time and still get the answer.

67 E Consider what gases are being collected in the beaker. Oxygen gas is flowing in from the reaction. Water vapor, H2O(g), is also entering the beaker from the evaporation of water. The pressure exerted by H2O(g) is equal to the vapor pressure of water. The total gas pressure in the beaker is, therefore, the sum of the pressure of oxygen gas collected and vapor pressure of water (at 25°C in this particular problem).

68 A If you draw the structure of acetylene, C2H2, you'll see that the carbon atoms must share three pairs of electrons to achieve stable octets.

H – C ≡ C – H

So C2H2 has a triple bond. None of the other molecules contains a triple bond. Since this is the strongest type of carbon-to-carbon bond, (A) is correct.

69 D Equilibrium is attained when the concentrations of all species become constant. The concentrations of N2O4 and NO2 stay the same from the 20-minute mark to the 25-minute mark. This means equilibrium was achieved before the 20-minute mark. Since the concentrations of N2O4and NO2 are different from the 15-minute mark to the 20-minute mark, equilibrium was not achieved at exactly 15 minutes from the start of the reaction; equilibrium was attained between 15 and 20 minutes after the start of the reaction.