MCAT General Chemistry Review
Part II Practice Sections
Practice Section 2
Directions: Most of the questions in the following General Chemistry Practice Section are organized into groups, with a descriptive passage preceding each group of questions. Study the passage, then select the single best answer to the question in each group. Some of the questions are not based on a descriptive passage; you must also select the best answer to these questions. In you are unsure of the best answer, eliminate the choices that you know are incorrect, then select an answer from the choices that remain.
PASSAGE I (QUESTIONS 1–9)
Swimming pools are filled with water containing a number of dissolved ions for the purpose of purification and maintenance of pH. One chemical that is added to pools, chlorine, is used to kill bacteria and harmful contaminants and can be added to pools in a number of ways. Calcium hypochlorite, Ca(OCl)2, is an inorganic chlorinating agent that contributes chlorine and calcium ions to the water.
Other chemicals and materials in swimming pools can also contribute calcium ions to the water. The concentration of calcium and other ions must be closely monitored so that the water does not become saturated with a particular compound. The solubility product constant, termed Ksp, describes the amount of salt in moles that can be dissolved in one liter of solution to reach saturation. No more salt can dissolve after reaching the point of saturation.
Plaster that lines swimming pools is a form of hydrated calcium sulfate, CaSO4. The calcium from chlorinating agents along with the calcium from plaster that lines swimming pools makes it necessary to monitor the concentrations of Ca2+ and SO42- to make sure that saturation is not reached. The following equation describes the dissociation of calcium sulfate in water:
CaSO4Ca2+ + SO42-
The Ksp value for the discussed dissociation reaction can be calculated by determining the values of [Ca2+] and [SO42-] in a saturated solution. If the Ksp value is known, the ion concentrations in swimming pools can be used with the Ksp value to predict whether or not the levels are at or near saturation.
1. If the Ksp of CaSO4 is calculated to be 4.93 × 10-5 at 25°C, what is the minimum amount of CaSO4 that can be added to 3.75 × 105 L of water to create a saturated solution?
A. 2.63 × 103 grams
B. 3.58 × 105 grams
C. 7.16 × 105 grams
D. 2.52 × 103 grams
2. Which of the following compounds, when dissolved in water, has the highest concentration of calcium for one mole of the compound?
A. CaCO3 (Ksp = 4.8 × 10-9)
B. CaF2 (Ksp = 3.9 × 10-11)
C. Ca3(PO4)2 (Ksp = 1 × 10-25)
D. Ca(IO3)2 (Ksp = 6.47 × 10-6)
3. The Ksp of CaSO4 can be calculated by determining the concentration of a saturated solution of CaSO4. The following graph shows the relationship between concentration and conductivity for CaSO4, which was determined by finding the conductance for four CaSO4 solutions of known concentration. Using the graph, estimate the concentration of a saturated solution that has a conductivity of 2.5 × 103 µS/cm and then calculate the experimental Ksp for a saturated solution of CaSO4:
A. 2.25 × 10-4 M2
B. 1.5 × 10-2 M2
C. 3 × 10-2 M2
D. 1.22 × 10-1 M2
4. In the experiment, a probe was used to measure the conductivities in each solution. The probe generates a potential difference between two electrodes and reads the current that is produced as a voltage. The computer then outputs the conductivity. If several different solutions were all heated from room temperature to 75° Celsius, how would the conductivity of the solutions change and how would the Ksp be affected?
A. The conductivity would increase and the Ksp would increase.
B. The conductivity would decrease and the change in Ksp would increase.
C. The conductivity would increase and the change in Ksp cannot be determined.
D. The conductivity would decrease and the change in Ksp cannot be determined.
5. Instead of using calcium hypochlorite to introduce chlorine into the water, the owner of a water park decides to bubble Cl2 gas into the pool. Will his decision affect the solubility of the plaster that is lining the pools at his water park?
A. Yes, the chlorine gas will increase the solubility of CaSO4 in the plaster as compared with Ca(OCl)2 because it will react with molecules of SO42- and shift the equilibrium to the right.
B. Yes, the chlorine gas will increase the solubility of CaSO4 in the plaster as compared with Ca(OCl)2 because Cl2 will not cause the same common ion effect that occurred with Ca(OCl)2.
C. Yes, the chlorine gas will decrease the solubility of CaSO4 in the plaster as compared with Ca(OCl)2 because the absence of Ca2+ from the Ca(OCl)2 will eliminate the common ion effect.
D. No, the chlorine gas will not change the solubility of CaSO4 in the plaster as compared with Ca(OCl)2.
6. Ca(OCl)2 contributes OCl- to the water, which acts to kill bacteria by destroying enzymes and contents of the cells through oxidation. In its ionic form, OCl- exists in the following equilibrium:
HOCl H+ + OCl-
In order for cleaning to occur properly, the pH must be at the right level to allow enough of the oxidizing agent, HOCl, to be present. If the pH is raised by the addition of sodium carbonate to the water, what will happen to the oxidizing power of the HOCl?
A. The higher pH will break the HOCl compound into single atoms and will eliminate its oxidizing power.
B. The pH cannot be raised due to the buffering system in the pool, and thus the oxidizing power of the chlorine will remain the same.
C. Fewer H+ ions will be present and the reaction will shift right. This will decrease the number of HOCl molecules and thus decrease the oxidizing power of chlorine.
D. A high pH will lower the concentration of H+ by associating H+ with OCl-. This will increase the number of HOCl molecules and increase the oxidizing power of chlorine.
7. Due to changes in climate and poor management of ion content in the water, the swimming pool has now become supersaturated with calcium sulfate. What combination of events could have caused this to occur?
A. Cooling of the pool followed by addition of calcium sulfate
B. Warming of the pool followed by addition of calcium sulfate
C. Addition of calcium sulfate followed by cooling of the pool and then subsequent warming of the pool
D. Warming of the pool followed by addition of calcium sulfate and then cooling of the pool
8. Water “hardness” refers to the content of calcium and magnesium in water. When referring to swimming pools, water hardness mainly refers to calcium. One way to measure the balance of ions is to use the Langelier saturation index. The Langelier saturation index is derived from a combination of the following two equilibrium equations.
Which of the following accurately expresses the combination of the two equilibrium equations in terms of [H+]?
9. Phenol red is the most widely used indicator to determine the pH of water in swimming pools. The pKa2 of phenol red is equal to 7.96. The acidic form of phenol red appears yellow and the basic form of phenol red appears red. In addition, the absorptivity (how strongly a species absorbs light) of the basic form is around three times greater than the acidic form. The color-changing region is indicated by an orange color. Due to the difference in absorptivity between different forms of phenol red, at what pH would the color change (to orange) be MOST likely to occur?
A. pH of 4
B. pH of 7.5
C. pH of 8.5
D. pH of 11
PASSAGE II (QUESTIONS 10–17)
Hydrogen is the first element of the periodic table. It contains one proton and one electron. According to one early model of the hydrogen atom developed in the early 20th century by Niels Bohr, that electron is found in any one of an infinite number of energy levels. These energy levels are sometimes called quanta, in that they can be described by a principal quantum number, n, that always has an integer value. As the electron moves from one energy level to another (n = 1 to n 8, or vice versa) it absorbs or emits some discrete quantity of energy accordingly. This quantity is directly proportional to the frequency of the light radiation that results from the energy change.
It took some time for atomic physicists to arrive at Bohr’s conclusions. They struggled to reconcile empirical data about light radiation from hydrogen atoms with their understanding that light photons moved and behaved as particles according to Newtonian mechanics. One discovery that led to Bohr’s quantum mechanics was a new quantitative interpretation of light emissions from hydrogen atoms. Hydrogen atoms emit light in characteristic patterns known as line spectra. These patterns are noncontinuous but predictable. In the early 1880s, Theodore Balmer derived a mathematical relationship between the energy emissions of a hydrogen atom and the wavelengths of light they radiated during transitions:
? = B[m2/(m2–22)] = B[m2/(m2–n2)], where B = 364.56 nm, m > 2, is the wavelength, n is equal to 2.
The spectrum he used, which is now known as the Balmer series, is illustrated below:
The Rydberg equation is a more general version of this equation that applies to all possible energy level transitions in a hydrogen atom. Physicists used the Rydberg equation to detect other series of energy transitions in other regions of the light spectrum. One such series is the Lyman series, which accounts for transitions from excited states to n = 1, the ground state.
10. What is the proper electron configuration of hydrogen in its elemental state?
D. None of the above
11. If an electron is promoted from n = 2 to n = 5, as Balmer observed, which of the following possibilities BEST describes the source of the line spectra observed?
A. A photon is absorbed.
B. A photon is emitted.
C. An electron is absorbed.
D. An electron is emitted.
12. What region of the light spectrum corresponds to the characteristic emissions in the Balmer series?
13. One Balmer spectral line, the n = 3 to n = 2 transition is a common reference point in astronomy for hydrogen gas emissions. The characteristic wavelength of this emission in the scientific literature is 656.3 nm. What color is this light emission?
D. The emission is not in the visible spectrum.
14. What name BEST describes the absorption line spectrum pictured below?
A. Lyman series
B. Balmer series
C. Bohr series
D. None of the above
15. Suppose a scientist tried to obtain a Balmer series with a sample of deuterium. How would this sample change the appearance of the emissions in the line spectrum?
A. Fewer emission lines
B. More emission lines
C. Same number of emission lines with split peaks
D. Same number of emission lines without split peaks (that is, no change in appearance)
16. At high resolution, some of the emissions in the Balmer series appear as doublets. Which of the following BEST explains this result, which was not predicted by any of the models in the passage?
A. The models in the passage do not account for relativistic effects.
B. The models in the passage do not account for high wavelengths.
C. The models in the passage do not account for the atomic number.
D. The models in the passage do not account for other particles in the atom.
17. When n > 6, the Balmer series features violet light emissions at wavelengths outside the range of the visible spectrum. Which of the following BEST accounts for this finding?
A. Energy levels are narrower as n 1 and wider as n 8.
B. Energy levels are wider as n 1 and narrower as n 8.
C. Energy differences between levels are larger as n 1 and smaller as n 8.
D. Energy differences between levels are smaller as n 1 and larger as n 8.
QUESTIONS 18–22 ARE NOT BASED ON A DESCRIPTIVE PASSAGE.
18. Which of the following pairs of particles would be accelerated in a particle accelerator?
A. Gamma ray and neutron
B. Gamma ray and beta particle
C. Beta particle and neutron
D. Alpha and beta particles
19. What volume of 0.5 M KOH would be necessary to neutralize 15 mL of 1 M nitrous acid?
A. 15 mL
B. 30 mL
C. 45 mL
D. 60 mL
20. Why does high, but not low, pressure cause a deviation from the ideal gas law?
A. Higher pressure decreases the interatomic distance to the point where intermolecular forces reduce the volume below that predicted by the ideal gas equation.
B. Low pressure increases the atomic radius of a gas making it more stable whereas high pressure compresses the gas particles decreasing their stability.
C. Low pressure does cause a significant deviation from the ideal gas law because the increased interatomic distance means that no particles ever collide.
D. Low pressure does cause a significant deviation because a low pressure implies a reduction in temperature via Charles’ law, which increases the power of intermolecular forces.
21. What type of molecular geometry is NOT able to result in a nonpolar structure?
B. Diatomic covalent
C. Trigonal planar
D. Square planar
22. A parent and daughter nucleus are isotypes of the same element. Therefore, the ratio of alpha to beta decays that produced the daughter nucleus must be which of the following?
PASSAGE III (QUESTIONS 23–30)
Many new consumer electronics and electric cars utilize a type of rechargeable battery that extracts its power from the movement of a lithium ion (Li+) between the cathode and the anode of a galvanic cell. In most cases, the anode is composed of graphite, the cathode is composed of a CoO2-complex, and the electrolyte contains a lithium salt in an organic solvent. Following are the half-reactions, where Li1-xCoO2 is the simplest form of the chemical formula Li(CoO2)1/(1-x) (which represents a complex of one lithium ion with several metal oxide molecules):
LiCoO2Li1-xCoO2 + xLi+ + xe- (cathode)
xLi+ + xe- + 6C LixC6 (anode)
The value of x is equal to the following ratio:
When the battery is fully charged, x equals 1. When the battery is fully discharged, x equals 0.
To study the change in a battery’s performance over time, a scientist repeatedly charged and discharged cell 1 while leaving cell 2 intact. After several cycles of charging and discharging, the energy-storage capacity of cell 1 deteriorated significantly faster than the capacity of cell 2. Upon further testing, cell 1 was found to contain approximately equal concentrations of lithium oxide and cobalt(II) oxide. The constituents of cell 2 were not analyzed. The scientist hypothesized that the deterioration of cell 1 was caused primarily by the conversion of integral cell components into lithium oxide and cobalt(II) oxide.
23. What is the net overall equation for the cell?
A. LiCoO2 + 6C LixC6 + Li1-xCoO2
B. LiCoO2 + xLi+ + 6C LixC6 + Li1-xCoO2
C. Li+ + 6C LixC6
D. LiCoO2 + xLi+Li1-xCoO2
24. Which of the following is TRUE about the overall potential of the cell when the battery is in use after a complete charge?
A. E°cathode + E°anode < 0
B. E°cathode + E°anode > 0
C. E°cathode + E°anode < 1
D. E°cathode + E°anode > 1
25. Which of the following identifies the oxidized and then the reduced species in the forward reaction?
A. Oxidized: Li+/ Reduced: Co4+
B. Oxidized: C/Reduced: Li
C. Oxidized: C x/6/Reduced: Co4+
D. Oxidized: Co3+/Reduced: C
26. Which of the following is TRUE about the equilibrium constant of the reaction?
A. The forward reaction exhibits a positive E°cell, which suggests a spontaneous process. Because discharging is spontaneous and charging is not, Keq is high during discharging.
B. An increasing value of x will push the cathode reaction to the left and the anode reaction to the right; therefore, charging and discharging will have no net effect on Keq.
C. Discharging is a spontaneous reaction, which requires reduction to occur at the cathode and oxidation to occur at the anode. Because this is only true for the reverse reaction, Keq is low during discharging.
D. Based on the information in the passage, it is impossible to predict the effects of charging and discharging on Keq.
27. A certain battery is equipped with a mechanism that calculates its remaining energy by approximating the concentration of various lithium-cobalt-oxygen complexes. The analysis finds that the predominant species in the battery are LiCoO2 and Li(CoO2)2. If the battery originally stored 100 J of potential energy, how much does it currently store?
A. 100 J
B. 50 J
C. 33 J
D. 0 J
28. Which of the following lithium species carry an oxidation number of +1?
I. Li from LiCoO2
II. Li from Li1-xCoO2
III. Li from LixC6
A. I only
B. II only
C. I and III only
D. I, II, and III
29. Which of the following BEST explains the appearance of lithium oxide and cobalt(II) oxide in cell 1?
A. A small number of lithium ions occasionally combined with LiCoO2 to produce lithium oxide and cobalt(II) oxide.
B. Because of the energy released by the system, a few LiCoO2 molecules decomposed into lithium oxide and cobalt(II) oxide every time the battery was used.
C. Various constituents of the cell combined with environmental oxygen to produce lithium oxide and cobalt(II) oxide.
D. Various constituents of the cell combined with water to produce lithium oxide and cobalt(II) oxide.
30. If the scientist’s hypothesis is correct, which of the following methods would be most likely to effectively measure the deterioration in the energy-storage capacity of a cell (like the one in cell 1)?
A. Determining the value of x for a fully charged battery
B. Determining the value of x for a fully discharged battery
C. Measuring the concentration of cobalt(II) oxide in a fully discharged battery
D. Measuring the concentration of LixC6 in a fully charged battery
PASSAGE IV (QUESTIONS 31–37)
Water is the most abundant liquid on Earth, covering over three-fourths of its surface. Compared with other liquids, it is quite extraordinary. Its chemical structure and resulting phase-change properties made the chances for the evolution of life on Earth a possibility. Due to their polarity, water molecules have the ability to form hydrogen bonds with one another and with other polar substances. As a result of these forces, water forms a crystalline lattice in its solid state, as depicted by the illustration below. The larger circles represent oxygen and the smaller circles represent hydrogen in the lattice.
Ammonia is very similar in structure to water. Because of this similarity, many biologists have wondered whether ammonia would be a suitable substitute for water in living systems. The ammonia molecule is composed of hydrogen atoms covalently bonded to nitrogen. As the oxygen in the water molecule has a slightly negative charge, so does the nitrogen atom in ammonia. Some scientists have argued that ammonia-based life could evolve on other planets in a similar manner as life developed on Earth. Others argue that ammonia’s heat of vaporization, 295 cal/g, is low compared with water, making it an unlikely candidate for the evolution of life. Modern science agrees that ammonia-based life on other planets will probably not be found to have evolved, if it exists, in the same manner as life on Earth.
31. One could infer from the passage that no form of life based on ammonia has yet been found because its
A. evaporation rate would be too high.
B. condensation rate would be too high.
C. rate of deposition would be too high.
D. rate of sublimation would be too low.
32. Had water not formed a crystalline lattice upon freezing and instead followed the common phase-change pathways of most other compounds, one could logically infer that
A. life could not have evolved in a liquid environment.
B. life would have evolved in a gaseous environment.
C. soils would hold greater amounts of liquid water.
D. soils would hold greater amounts of gaseous water.
33. When the water molecules shown in the previous illustration undergo sublimation, what BEST explains this phenomenon?
A. The attractive forces between the water molecules overcome the kinetic energy that keeps them apart.
B. The kinetic energy of the water molecules overcomes the attractive forces that keep them together.
C. The hydrogen bonds between the water molecules form at a more rapid rate in the solid phase.
D. The hydrogen bonds between the water molecules form at a more rapid rate in the liquid phase.
34. A change in which intrinsic property of water would MOST affect its polarity?
A. Atomic electronegativity
C. Intermolecular forces
35. How would one explain the negative slope of the water-solid equilibrium line in the phase diagram for water, shown below?
A. Water is more dense at 4° Celsius than at 0.
B. Water’s solid lattice collapses under pressure.
C. The triple point determines the slope of the phase-change line.
D. Liquid water is less dense than ice.
36. According to the passage, which of the following is TRUE about a molecule of water undergoing evaporation?
A. The water molecule has more energy than an ammonia molecule.
B. An average water molecule is moving faster than an average molecule of ammonia.
C. The water molecule has less energy than an ammonia molecule.
D. Both A and B
37. What would the addition of an ionic compound do to the lattice structure shown in the illustration above?
A. Collapse the structure
B. Enhance cohesive forces in the structure
C. Enhance crystallization
D. Both B and C
PASSAGE V (QUESTIONS 38–45)
There are many elements in the periodic table that play an integral role in the everyday functions of the human body. While the most obvious of these are carbon, hydrogen, nitrogen, and oxygen, we also have various essential uses for phosphorus. Its physical and chemical properties have made it the perfect candidate to play a role in the molecule that acts as the body’s primary location for short-term storage of energy, adenosine-5?-triphosphate (ATP).
Some metabolic syndromes can cause a phosphate ion to be drawn from the blood into the bones and teeth, where the majority of phosphorus exists. The ensuing deficiency of phosphate in the blood can lead to dysfunction of the brain and muscle tissue, which can cause death in severe cases. Scientists are attempting to cure these syndrome by designing a biologic molecule whose properties and actions are similar to those of phosphate. One way to do this is to find an element that has similar physical and chemical properties to phosphorus.
38. Which of the following is MOST likely to act as a stronger oxidizing agent than phosphorus?
39. Which of the following, on average, has more space between two nuclei placed side-by-side than phosphorus?
A. I only
B. I and II
C. II and III
D. I, II, and III
40. Which of the following is NOT a property of phosphorus, according to the classifications of types of elements?
A. Brittle in the solid state
B. Poor electrical conductivity
C. Does not show much luster
D. Is generally malleable
41. Which of the following is the correct electron orbital configuration of phosphorus?
42. Which of the following statements is TRUE about the density of alkali metals and alkaline earth metals?
A. Alkaline earth metals are less dense because they contain unfilled subshells.
B. Alkaline earth metals are less dense because their nuclei contain fewer neutrons.
C. Alkali metals are less dense because they contain fewer orbitals.
D. Alkali metals are less dense because they have a loosely bound electron in their outer shell.
43. Which of the following is NOT a correct characterization of the properties of halogens?
A. At room temperature, halogens naturally exist only in the gaseous and liquid states.
B. Halogens are highly likely to react with alkali metals.
C. Halogens can form stable ionic crystals with alkaline earth metals.
D. In their neutral form, halogens always have an outer shell of p5.
44. Which of the following contributes MOST to the malleability shown by transition elements?
A. Natural softness as compared to other metals
B. High electrical conductivity
C. Loosely held d-electrons
D. High melting points
45. Which of the following BEST explains why scientists closely examine metalloids when trying to find a biologic molecule to replace phosphorus?
A. Phosphorus is a metalloid.
B. Metalloids often behave as semiconductors.
C. Some metalloids exhibit similar bonding capabilities to phosphorus.
D. Metalloids exhibit flexibility in their properties so they can be manipulated easily.
PASSAGE VI (QUESTIONS 46–52)
Aerobic and anaerobic bacteria undergo different types of metabolism, and the properties of their metabolisms are unique. Some anaerobic bacteria are methane-producing bacteria. These bacteria have been studied to determine the relevance of their potential use in generating biological energy orbiogas.
These methane-producing bacteria typically feed off of animal manure or other natural waste. In the process of utilizing animal waste, manure is collected from different types of animals including swine and cows. The manure is separated by phase and contains proteins, carbohydrates, and fats; bacteria then break down components into fatty acids.1
Methanogens are the particular type of anaerobic bacteria that undertake the final steps of breaking down the fatty acids into simple products: methane and carbon dioxide. A common reactant is acetic acid, which breaks down according to the following reaction:
CH3COOH CH4 + CO2
Other fatty acids can be used as substrates for these methane-generating reactions, including propionate and butyrate. Methanogenic bacteria can also convert carbon dioxide and hydrogen gas to form methane and water. At the end of this process, which typically takes place at 95° Celsius, methane can be used to generate energy as an alternative to fossil fuels.
46. What type of reaction is presented in the passage?
A. Combination reaction
B. Single-displacement reaction
C. Decomposition reaction
D. Combustion reaction
47. If the reaction begins with 120 grams of acetic acid, what is the theoretical yield, in grams, of methane?
A. 32.05 grams
B. 64.1 grams
C. 40.92 grams
D. 29.12 grams
48. What piece of evidence would support the passage’s argument that the decomposition of fatty acids can create energy serving as an alternative to fossil fuel?
A. Anaerobic bacteria can break down fatty acids efficiently at room temperature.
B. Methane gas can be compressed and transported.
C. The reaction generates gaseous products.
D. The reaction is exothermic.
49. What formula BEST demonstrates how to calculate the number of grams of acetic acid necessary to produce 88.02 grams of carbon dioxide?
50. One form of acetic acid, which is typically used as a salt, is called acetate. Sodium acetate can react with other chemical compounds in solution, an example of which is demonstrated below.
What is the correct net ionic equation for this reaction?
51. As described in the passage, methanogenic bacteria can utilize hydrogen gas to produce methane. At standard temperature and pressure, if there are 3 liters of hydrogen gas and 2 liters of carbon dioxide available to the bacteria, what would be the theoretical yield, in moles, of methane?
A. 0.134 moles
B. 0.0893 moles
C. 0.0335 moles
D. 0.536 moles
52. What is the percent yield if 8.02 g of methane is formed from the reaction of 50 liters of hydrogen gas, with excess carbon dioxide at standard temperature and pressure?