MCAT General Chemistry Review

Part II Practice Sections

Practice Section 3

Time—70 minutes

QUESTIONS 1–52

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.

413

PASSAGE I (QUESTIONS 1–8)

The blood-brain barrier is a unique part of the human nervous system. Endothelial cells lining blood vessels in the central nervous system (CNS) are more tightly attached to one another than in other parts of the human body. As a result, there is limited permeability of both small and large molecules from the circulation into the cerebrospinal fluid (CSF).

414

These tightly sealed endothelial cells have both advantages and disadvantages in the human system. The CNS is a fragile, essential part of the human body, and the endothelial cells serve as a barrier. Multiple characteristics of any given molecule affect its permeability: its polarity, size, weight, charge, and degree of protein binding in the blood. Nonpolar molecules pass more effectively from the bloodstream into the CSF. Smaller particles, such as water, and small, charged particles will also move with varying ease across this barrier. Water moves freely, but charged ions can take hours to equilibrate between the systemic circulation and the CSF.

When disease afflicts the CNS, it is necessary to deliver drugs to the cerebrospinal fluid for delivery into the tissues of the brain and spinal cord. On the other hand, some extremely effective chemotherapeutic agents, such as cisplatin, are beneficial when they do not cross the blood-brain barrier because they are neurotoxic when they penetrate the CNS. Alternatively, when beginning general anesthesia for a surgical procedure, it is essential that anesthetic agents penetrate from the systemic circulation into the CSF to alter consciousness and systemic muscle tone during the procedure.

1. Based on the passage, which of the following characteristics would be essential for any pharmaceutical intended for use as a general anesthetic?

A. The molecule should be nonpolar.

B. The molecule should be directly delivered to the CNS without going through the systemic circulation first.

C. The molecule should be polar.

D. The molecule should be slow-acting.

2. What can be logically inferred from the passage about charge and its effect on a molecule’s permeability of the blood-brain barrier?

A. Charged molecules are more likely to associate with one another tightly in the blood stream, inhibiting diffusion into the CSF.

B. Uncharged molecules are more likely to be able to diffuse between endothelial cells.

C. Uncharged molecules are less likely to be transported through endothelial cells.

D. Charged molecules are more soluble in the bloodstream than in the CSF.

3. What can be logically inferred from the passage about the role of the blood-brain barrier in supporting human life?

A. A permeable CNS is essential in allowing diffusion of nutrients from the peripheral circulation into the CNS.

B. The micro-environment of the CNS is similar to that of the systemic circulation.

C. The blood-brain barrier limits the flow of damaged or infected cells from the CSF into the systemic circulation.

D. The blood-brain barrier adaptively protects the CNS from toxins or other possible insults originating in the systemic circulation.

4. Based on the information in the passage, what type of intermolecular force has the MOST influence on molecules that pass easily through the blood-brain barrier?

A. Ion-dipole interactions

B. Dipole-dipole interactions

C. Hydrogen bonding

D. Dispersion forces

5. Which of the following statements is NOT true when relating formal charge with permeation across the blood-brain barrier?

I. A formal charge of zero guarantees permeability through the blood-brain barrier.

II. A negative formal change on one or more atoms in a molecule will improve its permeability of the blood-brain barrier.

III. Two molecules, both with formal charges of zero, will be equally permeable through the blood-brain barrier.

A. I only

B. III only

C. II and III only

D. I, II, and III

6. Cisplatin, a commonly used chemotherapeutic agent, is PtCl2(NH3)2. What type of bond forms between each of the NH3 groups and the central platinum?

A. Coordinate covalent bond

B. Polar covalent bond

C. Nonpolar covalent bond

D. Ionic bond

7. Phenytoin, shown below, is an anti-seizure medicine. It is one of many drugs that is actively transported out of the CNS by cellular transporters. What would be the best estimate of the geometry around the central carbon to which the arrow points?

415

A. Square planar

B. Tetrahedral

C. Trigonal pyramidal

D. Octahedral

8. Which of the following BEST describes the relationship between resonance structures and molecular polarity?

A. The most stable resonance structures maximize polarity.

B. If a molecule has more than one important resonance structure, it is more likely to be a polar molecule than another molecule without such resonance structures.

C. The most important resonance structures spread out and minimize formal charge.

D. Resonance structures will counterbalance the natural polarity of a bond.

PASSAGE II (QUESTIONS 9–16)

The human body is a dynamic system that has to deal with significant environmental threats on a daily basis. One type of threat is from the effects of reactive oxygen species (ROS). ROS are ions or small molecules containing oxygen that have unpaired valence shell electrons. The superoxide anion, O2-, is a toxic threat, becoming lethal at intracellular levels of just 1 nM. It spontaneously forms O2 and H2O2, but is also able to react with NO to form peroxynitrite. Peroxynitrite can cause extreme cellular damage. The enzyme NADPH oxidase produces the superoxide anion in the body to combat invading microorganisms. Because of the threat that it poses in such small quantities, the body has developed ways to dispose of this chemical.

The superoxide anion puts the concept of compartmentalization on display. It would be a waste of energy to both produce and destroy superoxide in the same cell, so it is only produced in phagocytes (immune cells that ingest infectious agents), and is broken down in any other cell of the body. In two steps, the enzyme superoxide dismutase (SOD) uses iron or other metals to create oxygen and hydrogen peroxide from superoxide and hydrogen ions:

Step 1: Fe3+- SOD + O2-image Fe2+ - SOD + O2

Step 2: Fe2+- SOD + O2- + 2H+image Fe3+ -

SOD + H2O2

A graduate student at a local university was given the task of determining the kinetics of this reaction. Her results are shown in table 1 below:

416

Table 1

The potential energy diagram for the reaction is shown in figure 1 below:

417

Figure 1

FOR QUESTIONS 9–14, ASSUME BOTH STEPS OF THE REACTION ARE IRREVERSIBLE.

9. What is the order of H+ in step 2 of the above reaction?

A. 0

B. 1

C. 2

D. 3

10. What is the rate of step 2 of the reaction if the following concentrations of reactants exist? Assume the rate constant, k = 0.50.

[H+] = 2 M, [O2-] = 2 M

A. 2.5 M/sec

B. 4 M/sec

C. 8 M/sec

D. 10 M/sec

11. What function might Fe2+ - SOD play in the overall reaction?

A. A substance used to create a product in the reaction

B. A substance that is created in the reaction

C. A substance that increases the rate of the reaction

D. A short-lived, unstable molecule in the reaction

12. Which of the following is NOT true when describing the kinetics of the previous overall reaction?

A. The rate of the reaction is proportional to the number of collisions between reacting molecules.

B. In some effective collisions, all of the colliding particles do not have enough kinetic energy to exceed activation energy.

C. A transition state is formed when old bonds are breaking and new bonds are forming.

D. The activated complex has greater energy than either products or reactants.

13. What section of the diagram in figure 1 represents the forward activation energy?

A. A

B. B

C. C

D. D

14. What section of the diagram in figure 1 represents the enthalpy change during the reaction?

A. A

B. B

C. C

D. D

FOR QUESTIONS 15–16, ASSUME BOTH STEPS OF THE REACTION ARE REVERSIBLE.

15. What is the equilibrium constant for the overall reaction in the passage and will an increase in pressure raise or lower the equilibrium constant? Assume [O2-] = 3 M, [H+] = 1 M, [H2O2] = 1 M, [O2] = 2 M.

A. 0.22; raise

B. 0.27; lower

C. 2.2; raise

D. 2.7; lower

16. What would BEST explain the equilibrium shift to the right in reaction 2?

A. Increase in volume

B. Addition of product

C. Addition of reactant

D. Decrease in temperature

QUESTIONS 17–21 ARE NOT BASED ON A DESCRIPTIVE PASSAGE.

17. Latent heat flux is the loss of heat by the surface of a body of water caused by evaporation. To determine the latent heat flux over the Atlantic Ocean, one would need to know

A. imageH fusion of water.

B. imageH vaporization of water.

C. imageH sublimation of water.

D. imageH ionization of water.

18. If the pKa of a weak acid is 5, the pH will be 6

A. when the concentration of dissociated acid is one-tenth the concentration of undissociated acid.

B. when half the acid is dissociated.

C. when the concentration of dissociated acid is ten times the concentration of undissociated acid.

D. only after a base has been added.

19. What compound would NOT be considered an electrolyte?

A. AgCl

B. CaO

C. LiI

D. HBr

20. Aluminum has a lower electronegativity than iron, but reacts extremely slowly with oxygen in moist air because of a hard, protective aluminum oxide coat that protects all exposed surfaces. Under which of the following conditions would aluminum be more readily eroded?

A. Immersed in a solution of HCl

B. Immersed in a bath of hot sodium metal

C. Immersed in a solution of NH3

D. Immersed in a solution of NaOH

21. Two gases, X and Y, are combined in a closed container. At STP, the average velocity of a gas A molecule is twice that of a gas B molecule. Gases A and B are most likely which of the following?

A. He and Ar

B. He and Kr

C. Ne and Ar

D. Ne and Kr

PASSAGE III (QUESTIONS 22–29)

Dry ice forms when carbon dioxide gas is cooled to -78° C at atmospheric pressure. After becoming solid, it reforms gas when heat is added as shown in the reversible reaction below:

CO2 (solid, -78° C) + heat (120kJ/mol) image CO2 (gas, 25° C)

An experiment was done to test the change, over three days, in a block of dry ice placed in a rigid container at room temperature at 1 atmosphere of pressure. The container was closed to the outside environment for the duration of the experiment. The only components in the container were the dry ice and air (g). No liquid in the container was detected over the three-day period. The apparatus and results of the experiment are shown in Figure 1 below.

418

Figure 1

As a solid, carbon dioxide has many uses, not the least of which is cooling its surroundings. This transfer of energy is a main method by which coolants operate in many mechanical devices. The phase diagram for carbon dioxide is a major reason for its unique behaviors. The phase diagram for carbon dioxide is shown in Figure 2 below.

419

Figure 2

22. Which of the following BEST describes the equation in the passage?

A. Evaporation

B. Condensation

C. Fusion

D. Sublimation

23. Referring to figure 2, if liquid carbon dioxide were subject to increasing pressure at a constant temperature, it would

A. become solid.

B. become gaseous.

C. gain kinetic energy.

D. lose kinetic energy.

24. It can be inferred from the results of the experiment that the air in the container

A. lost kinetic energy.

B. gained kinetic energy.

C. gained volume.

D. lost volume.

25. When the dry ice molecules shown undergo phase changes, which of the following is a likely cause?

A. The attractive forces between the carbon dioxide molecules overcome the kinetic energy that keeps them apart.

B. The kinetic energy of the carbon dioxide molecules overcomes the attractive forces that keep them together.

C. The hydrogen bonds between the carbon dioxide molecules form at a more rapid rate in the solid phase.

D. The hydrogen bonds between the carbon dioxide molecules form at a more rapid rate in the liquid phase.

26. The process shown in the experiment from the passage was

A. endothermic, and the dry ice gained potential energy.

B. endothermic, and the dry ice lost potential energy.

C. exothermic, and the dry ice gained potential energy.

D. exothermic, and the dry ice lost potential energy.

27. If the experiment from the passage were allowed to continue until all the carbon dioxide changed phase, one could logically predict that the air in the container would have

A. increased in pressure.

B. decreased in volume.

C. become a solid.

D. increased in temperature.

28. In the heating/cooling curve for carbon dioxide shown below, what represents the location of the phase change described in the passage?

420

A. A

B. B

C. C

D. None of the above

29. In the phase diagram for carbon dioxide shown below, what represents the phase change shown in the experiment in the passage?

421

A. A

B. B

C. C

D. None of the above

QUESTIONS 30–36 ARE NOT BASED ON A DESCRIPTIVE PASSAGE.

30. Which of the following will result in a negative free energy change for a reaction?

A. The enthalpy change is negative.

B. The entropy change is positive.

C. The enthalpy change is negative and the entropy change is negative.

D. The enthalpy change is negative and the entropy change is positive.

31. Compared to the atomic radius of calcium, the atomic radius of gallium is

A. larger, because increased electron charge requires that the same force be distributed over a greater number of electrons.

B. smaller, because gallium gives up more electrons, decreasing its size.

C. smaller, because increased nuclear charge causes the electrons to be held more tightly.

D. larger, because its additional electrons increases the volume of the atom.

32. Under which conditions would water vapor demonstrate behavior closest to an ideal gas?

A. High pressure, low temperature

B. Low pressure, low temperature

C. High pressure, high temperature

D. Low pressure, high temperature

33. If the pressure of an ideal gas in a closed container is halved while the volume is held constant, the temperature of the gas

A. decreases by a factor of 2.

B. decreases by a factor of 4.

C. remains the same.

D. increases by a factor of 4.

34. “Greenhouse gases” are gases that will absorb IR radiation and trap energy between the Earth and the atmosphere. CO2 and H2O both strongly absorb radiation and are thus considered greenhouse gases, while N2 and O2 do not. One quality of greenhouse gases is that

A. they are composed of polar molecules.

B. they have a permanent dipole moment.

C. they experience hydrogen bonding.

D. they have polar covalent bonds.

35. What element contains unpaired electrons in its most common ionized state?

A. Fluorine

B. Aluminum

C. Zinc

D. Iron

36. Which of the following is the correct electron configuration for chromium in the ground state?

A. [Ar]3d45s2

B. [Kr]4d55s1

C. [Ar]4s14p5

D. [Ar]3d54s1

PASSAGE IV (QUESTIONS 37–45)

Decompression sickness involves symptoms that arise from exposure to a rapid decrease in ambient pressure. Decompression sickness can occur in multiple scenarios of decreased pressure and is most prevalent when divers return to the surface of water after a deep dive. If a diver ascends quickly and does not carry out decompression stops, gas bubbles can form in the body and create a multitude of adverse symptoms.

A diver experiences an increase in pressure when submerged many feet under water. Inert gases in the high-pressure environment dissolve into body tissues and liquids. When a diver comes back to the water’s surface and the pressure decreases, the excess gas dissolved in the body comes out of solution. Gas bubbles form if inert gas comes out of the body too quickly. These bubbles are unable to leave through the lungs and subsequently cause symptoms such as itching skin, rashes, joint pain, paralysis, and even death.

At sea level the pressure exerted on one square inch is equal to 14.7 pounds, or 1 atm. In water, an additional 1 atm of pressure is exerted for every 33 feet (about 10 m) below sea level. In addition to the decompression sickness, there are other conditions of which divers must be aware that arise from specific gases as a result of the high-pressure environment. For example, increased concentrations of nitrogen in the body lead to nitrogen narcosis. A diver with nitrogen narcosis feels intoxicated and experiences loss of decision-making skills due to nitrogen’s anesthetic quality. The table below provides a list of gases and their corresponding solubility constants in water at 298 K.

Gas

k (M torr –1)

CO2

4.48 × 10–5

O2

1.66 × 10–6

He

5.1 × 10–7

H2

1.04 × 10–6

N2

8.42 × 10–7

37. What theory could be used to determine the amount of oxygen that is dissolved in water at sea level?

A. Henry’s law

B. Boyle’s law

C. Raoult’s law

D. Le Châtelier’s principle

38. What is the solubility (g/L) of N2 in water (25° C) when the N2 partial pressure is 0.634 atm?

A. 3.19 × 10-1 g/L

B. 1.5 × 10-2 g/L

C. 1.14 × 10-2 g/L

D. 1.5 × 10-5 g/L

39. Helium is mixed with oxygen in the scuba tanks of divers in order to dilute the oxygen. Why is helium chosen over other gases for this purpose?

A. It is not a diatomic gas.

B. It is less soluble in aqueous solutions and so does not dissolve in body tissues and fluids.

C. It can react with other gases that may dissolve in the body to reverse gas bubble formation.

D. It is present only in trace amounts in water.

40. A scuba tank is filled with 0.32 kg O2 that is compressed to a volume of 2.8 L. If the temperature of the tank equilibrates with the water at 13° Celsius, what is the pressure inside the tank?

A. 111 atm

B. 83.9 atm

C. 54.6 atm

D. 290 atm

41. Which of the following would you recommend for a diver suffering from decompression sickness?

A. Administration of helium gas

B. Administration of a gas and air mixture, which contains 50 percent nitrous oxide

C. Confinement in a hypobaric chamber

D. Confinement in a hyperbaric chamber

42. The underwater environment in the world’s oceans is rapidly changing. Recent years have seen drastic shifts in the ecosystem due to human activity and its impact on the environment. Many populations of fish that rely heavily upon oxygen are declining at extraordinary rates, whereas other ocean species that can survive in oceanic regions of oxygen-depletion are on the rise. What is the most likely explanation, based on scientific theory, for the decline in dissolved oxygen in the world’s oceans?

A. Carbon dioxide pollution has increased ocean acidity.

B. A new species of predator shark preys on fish in oxygen-rich regions.

C. The average temperature of the oceans is rapidly increasing.

D. Increased rainfall has added water to oceans without adding more oxygen.

43. A scuba tank contains 0.38 kg of oxygen gas under high pressure. What volume would the oxygen occupy at STP?

A. 0.27 L

B. 35 L

C. 266 L

D. 11 L

44. At 1 atm, the solubility of pure nitrogen in the blood at normal body temperature (37° C) is 6.2 × 10-4 M. If a diver is at a depth where the pressure is equal to 3 atm and breathes air (78% N2), calculate the concentration of nitrogen in the diver’s blood.

A. 1.3 × 10-3 M

B. 1.4 × 10-3 M

C. 1.5 × 10-5 M

D. 1.9 × 10-3 M

45. Consider two scuba tanks at sea level and 25° C. Tank 1 is filled with oxygen, and tank 2 is filled with a mixture of oxygen and helium. Will there be a difference in the root-mean-square velocities between these two tanks?

A. Yes, tank 2 has a higher root-mean-square velocity.

B. Yes, tank 1 has a higher root-mean-square velocity.

C. No, they will have the same root-mean-square velocity.

D. The root-mean-square velocity cannot be calculated for the tanks.

PASSAGE V (QUESTIONS 46–52)

Sodium fluoride is used in toothpastes to reduce the virulence of bacteria that cause dental caries, also known as cavities. Most U.S. residents are exposed to sodium fluoride, and its use has been correlated with a decline in the incidence of dental caries in most of the population. Although there is debate over the mechanism by which sodium fluoride acts to reduce dental caries, it has been established that the fluoride ion is the main contributor to its efficacy.

Fluoride is the ionic form of the element fluorine. The fluoride ion has a high degree of electronegativity and so holds a negative charge in solution. It thus forms relatively stable bonds with positive ions such as H+ and Na+. Fluoride inhibits carinogenic bacteria from metabolizing carbohydrates and thus prevents subsequent production of acid in the oral cavity. The decrease of acidity reduces erosion of tooth enamel, which would otherwise lead to fissures and irregular surface changes in the tooth.

In a variety of laboratory studies, certain types of Streptococci bacteria, a main culprit in the formation of caries, are adversely affected when exposed to fluoride ion concentrations of varying levels. In particular, it was found that Streptococcus mutans, the more virulent species of the Streptococci, produces less acid when exposed to fluoride than Streptococcus sobrinus, the less virulent form. The reasons for the link of reduced acid production to fluoride levels are still unclear.

46. Based on the passage, which of the following can be definitively stated about the action of fluoride ions on oral health?

A. F- prevents bacteria from forming dental caries.

B. F- kills populations of bacteria that cause dental caries.

C. F- is related to killing populations of bacteria that form dental caries.

D. F- is related to less acid production and reduces the risk of dental caries.

47. Based on the passage, what is a possible mechanism by which fluoride could act upon Streptococci bacteria to reduce their production of acid?

A. Fluoride adds enamel to the developing tooth structure.

B. Fluoride fills and closes fissures within the enamel topography.

C. Fluoride adds electrons to bacterial respiration reactions.

D. Fluoride pulls electrons from bacterial respiration reactions.

48. What property contributes to the high electronegativity found in the fluorine atom?

A. Small atomic radius

B. Small number of protons in the nucleus

C. Large number of electrons in the orbit

D. Large number of electron shells in the orbit

49. Which of the following is the correct electronic structure notation for fluoride?

A. 1s22s22p5

B. 1s22s22p4

C. [He]2s22p6

D. [Ne]2p6

50. According to Heisenberg, what can be accurately, quantitatively determined in a neutral atom when the location of the electron is found?

A. Electron momentum

B. Velocity of electron

C. Mass of electron

D. None of the above

51. What is the effective nuclear charge on the outermost electron in fluoride?

A. 9

B. 2

C. 7

D. 8

52. What electrons are most available for bonding in the fluoride ion?

A. 3s

B. 2p

C. 1s

D. 2s