Kaplan MCAT General Chemistry Review

When atoms or molecules are in close proximity for an extended period of time, there is a substantial probability that they will interact with one another. The reaction suggested in (A) would require reduction of Co³⁺ to Co²⁺, which is not unlikely to occur in small quantities. Though the passage does not directly indicate that this was the mechanism of production of lithium oxide and cobalt(II) oxide, it is more likely than any of the other answer choices. It would be incorrect to dismiss the possibility of decomposition, but the reaction suggested in (B) (LiCoO₂ Li₂O + CoO) is stoichiometrically impossible. (C) is incorrect because interaction with environmental oxygen would affect cell 2 just as much as it would affect cell 1. (D) is incorrect because we have no evidence suggesting that any water is present in the system; the passage clarifies that organic solvents are used.

30. C

The scientist believes that battery deterioration is caused primarily by the formation of cobalt(II) oxide and lithium oxide in equal quantities. Therefore, increased amounts of cobalt(II) oxide suggest a decrease in battery capacity. (A) and (B) are incorrect because x is a ratio that is unaffected by the energy storage capacity of a cell. (D) is incorrect because, although a decrease in concentration of Li_xC₆ would suggest a decrease in cell capacity, most of the deterioration is caused by the conversion of LiCoO₂ to Li₂O and CoO₂.

PASSAGE IV

31. A

Ammonia’s heat of vaporization is given in the data and described as low compared with that of water. Thus, its evaporation rate must also be high compared to that of water so life could not have evolved in a liquid ammonia environment as life on earth evolved in a liquid water environment. The phases in (B), (C), and (D) would matter less for life’s evolution from a liquid environment; they are also incorrect applications of the data from the passage.

32. A

Solid water would have been most dense if it were like other substances, and thus sunk to the bottom of any liquid system. Liquid systems would have been frozen from the bottom up, and life could not have evolved in the liquid phase of a watery environment. (B), (C), and (D) do not address this “what-if ” scenario.

33. B

It is the kinetic energy of the molecules that moves them further apart to allow a phase change from solid to gas, as described in the question. The same process would dictate a phase change from solid to liquid, or liquid to gas.

34. A

The electronegativities of the atoms comprising a molecule determine the polarity of that molecule. None of the other choices will make as significant a contribution. (C) is the next most logical answer, but is less correct because forces emanate from the intrinsic nature of electronegativity within the water molecule, not between.

35. B

The crystalline lattice formed for a water-solid uniquely collapses under pressure to become a liquid. (A) is a true statement, though it doesn’t directly address the question.

36. D

Both (A) and (B) are true. Water has a higher heat of vaporization than ammonia and therefore evaporates at a higher temperature. Temperature is a measurement of average kinetic energy, so high temperature means higher average kinetic energy; the molecules are also moving faster.

37. A

Ions dissolved in the lattice break existing intermolecular attractive forces. This process interferes with the formation of a crystal lattice in ice, which explains why ice melts when salt is added.

PASSAGE V

38. C

Asking for the strongest oxidizing agent is equal to asking for the element with the highest electronegativity. Electronegativity increases as we move from the left to the right and from the bottom to the top of the periodic table. Therefore, you are looking for an element that is above and to the right of phosphorus. (C), oxygen, fits this description.

39. B

You’re being asked which elements have a larger atomic radius than phosphorus. Atomic radius increases from right to left and from top to bottom of the periodic table. Therefore, you are looking for elements that are below and to the left of phosphorus. Both K and Pb fit this description, so (B) is the correct answer.

40. D

To answer this question, you must know that phosphorus is a nonmetal. The other answer choices describe properties of nonmetals. (D) describes a property of metals, so it is the correct answer.

41. B

(B) is the correct full electron configuration. (C) places the 3s-electrons in the 3p-orbital. (D) is incorrect because phosphorus does not have a 3d-orbital.

42. D

(A) and (B) are incorrect because alkaline earth elements are generally more dense than alkali metals. (C) is incorrect because alkali metals contain the same number of orbitals as the alkaline earth element in the corresponding row. (D) is a true statement.

43. A

Halogens can naturally exist in the gaseous, liquid, and solid states (iodine).

44. C