Kaplan MCAT General Chemistry Review

Mixtures that have a higher vapor pressure than predicted by Raoult’s law have stronger solvent–solvent and solute–solute interactions than solvent–solute interactions. Therefore, particles do not want to stay in solution and more readily evaporate, creating a higher vapor pressure than an ideal solution. Two liquids that have different properties, like hexane (hydrophobic) and ethanol (hydrophilic, small), would not have many interactions with each other to cause positive deviation. (B) and (C) are composed of liquids that are similar to one another and that would not show significant deviation from Raoult’s law because they neither attract nor repel one another. (D) contains two liquids that would interact well with each other, causing a negative deviation from Raoult’s law. When attracted to one other, liquids prefer to stay in liquid form and have a lower vapor pressure than predicted by Raoult’s law.

10. A

Dissolution is governed by enthalpy and entropy, which are related by the equation G°_soln = H°_soln - °_soln. The cooling of the solution indicates that heat is used up in this bond-breaking reaction. In other words, dissolution is endothermic, and H is positive. The reaction is occurring spontaneously, so G must be negative. The only way that a positive H can result in a negative G is if entropy, S, is a large, positive value. If has a larger absolute value than H, G will be negative. Conceptually, that means that the only way the solid can dissolve is if the increase in entropy is great enough to overcome the decrease in enthalpy. (B) is incorrect because it is clearly stated in the question stem that KCl dissolves. (C) is incorrect because S°_soln must be positive in order for KCl to dissolve. Finally, (D) is incorrect because solute dissolution would cause boiling point to elevate, not depress. Additionally, it is not a piece of evidence that could be found simply by observing the beaker’s temperature change.

11. B

The equation to determine the change in boiling point of a solution is as follows: T_b = m_b(K_b). m_b is the molality of the solution, and K_b is the boiling point elevation constant. In this case, the solvent is always water, so K_b will be the same for each solution and we don’t need it to find our answer. What we do need to know is how many particles dissociate from each original species. This is referred to as the van’t Hoff factor (i) and is multiplied by our molality to demonstrate an effective molality. We’ll use effective molality values to determine which will cause the greatest change in boiling point.

It looks like we need to decide between iron (III) nitrate and acetic acid. No more calculations are necessary because the fact that acetic acid is a weak acid tells us that only a few particles will dissociate into H⁺ and acetate^-. So the x that we are curious about is most likely closer to 1.09 than 2.18. We can certainly conclude that it will be less than 2.16. Iron (III) nitrate will have the largest effect on boiling point.

12. C

The solubility of gases in liquids is directly proportional to the atmospheric pressure. Therefore, we should expect a decrease in solubility upon experiencing the decreased atmospheric pressure in Denver. Because 0.800 atm is 80% of the pressure at sea level (1 atm), oxygen’s solubility will be 80% of 1.25 × 10^-3, which yields 1.00 × 10^-3 M. (A) is a miscalculation, (B) suggests that pressure and solubility are inversely related, and (D) implies that atmospheric temperature will not affect solubility.

13. C

30 ppb of Pb²⁺ is equivalent to 30 grams of Pb²⁺ in 10⁹ grams of solution. Water’s density will help us convert from mass to volume. Dividing by the molar mass of Pb²⁺, 207 g/mole, will result in Pb²⁺’s molarity, 1.4 × 10^-7 M.

14. D

An electrolyte is a molecule that dissociates into free ions and behaves as an electrically conductive medium. Number I, NaF, is an electrolyte because it dissociates to form the ions Na⁺ and F^-. Number II, glucose, is a nonelectrolyte because it has a ring structure that dissolves but does not dissociate. Number III, CH₃OH, has a pKa of approximately 15 and is not likely to ionize in solution. Therefore, it is not an electrolyte. Number IV, acetic acid, is a weak acid (pKa = 4.7) and also a weak electrolyte because it will partially ionize in solution.

15. D

A colligative property depends solely upon the number of molecules and disregards the identity of the molecules. (A), (B), and (C) are all properties based on the composition of a solution, determined by the number of molecules that are dissolved in the solution. (D), the entropy of dissolution, will depend on the chemical properties of the substance, such as charge density and electron affinity. Therefore, the entropy of dissolution is not a colligative property.

16. D

The solubility of AgBr can be solved by using the K_sp value given in the equation.

AgBr Ag⁺ + Br^-

K_sp = [Ag⁺][Br^-] = 7.7 × 10^-13