Kaplan MCAT General Chemistry Review

(B) has the lowest heat required for vaporization, which by definition determines the boiling point or the lowest temperature required for phase change from liquid to gas. All of the other answer choices have higher heats of vaporization, as seen in the table associated with the question.

9. B

The miscibility of two liquids strongly depends on their polarities. A polar liquid can mix with another polar liquid, and a nonpolar liquid can mix with another nonpolar liquid. In general, however, polar and nonpolar liquids are not miscible with each other. (A), molecular size, and (C), the density of a liquid, do not directly affect the miscibility (although (C) should remind you that two immiscible liquids will form separate layers, with the denser liquid on the bottom). Thus, (B) is the only correct choice.

10. B

The bonds between different metal atoms in an alloy are much weaker than those between the atoms in pure metals. Therefore, breaking these bonds requires less energy than does breaking the bonds in pure metals. The more stable the bonds are in a compound, the higher the melting and freezing points. They tend to be lower for alloys than for pure metals. Alternately, an alloy can be looked at as a solid solution; impurities lower the melting point.

11. C

The osmotic pressure () of a solution is given by = iM RT. At STP, T = 273 K. R, the ideal gas constant, equals 8.2 × 10^-2 L • atm/K • mol. To determine the molarity, find the formula weight of NaCl from the periodic table; FW = 58.5 g/mol. The number of moles in the solution described is

However, because NaCl is a strong electrolyte, it will completely dissociate in aqueous solution, yielding 4 moles of particles per liter of solution (i.e., 2 moles of Na⁺ and 2 moles of Cl^-). Thus,

? = (4 mol/L)(8.2 × 10^-2 L •atm/K •mol)(273 K) = 89.54 atm

Remember that colligative properties depend on the number of particles, not their identity.

Chapter 9: Solutions

What do first aid instant cold packs and sweet tea have in common? Not much, you might think, but both, in fact, demonstrate the same principles of solution chemistry. Instant cold packs contain two compartments, one holding water and the other ammonium nitrate. When you bend the pack, the two compartments break open, allowing the ammonium nitrate to dissolve into the water. Sweet tea is made by dissolving a large amount of sugar into strongly brewed tea—the sweetest of the sweet teas may approach 22 brix, which means that in every 100 grams of tea, there are 22 grams of sugar. (As a point of reference, this is about twice the sweetness of regular cola.) When the ammonium nitrate or the sugar dissolves in the water to produce the respective solutions, three events must take place. First, the intermolecular interactions between the molecules of ammonium nitrate (or between the sugar molecules) must be broken. Second, the intermolecular interactions between the molecules of water must be broken. Third, new intermolecular interactions between the ammonium nitrate and the water molecules (or between the sugar and the water) must be formed.

We know from our discussion in Chapter 3, Bonding and Chemical Interactions, that energy is required to break these intermolecular interactions and that energy is released when new ones are formed. For the creation of both the ammonium nitrate and sugar solutions, more energy is needed to break the original intermolecular interactions than is released when new intermolecular interactions are formed. Thus, the creation of both of these solutions is an endothermic process. Now, the formation of the ammonium nitrate solution is much more endothermic than the formation of the sugar solution. And this is why ammonium nitrate is useful in instant cold packs. When it dissolves in water, the system must absorb an amount of energy equal to 6.14 kcal/mol of ammonium nitrate. The heat is absorbed from the surrounding environment, and the pack feels cool to the touch.

Although the dissolution of sugar into water is not as strongly endothermic, we nevertheless have an intuitive understanding of the process’s endothermicity because we all know that the easiest way to dissolve lots of sugar into water (such as tea or coffee) is to heat up the water and then add the sugar. Because heating the water increases the solubility of sugar, it must be that the dissolution of sugar into water is an endothermic process (think application of Le Châtelier’s principle and changes in temperature from Chapter 5, Chemical Kinetics and Equilibrium).

In this chapter, our focus will be on the characteristics and behaviors of solutions. We have already begun our consideration of solutions in the previous chapter with our review of the colligative properties. We will now continue that review by examining the nature of solutions, the formation of aqueous solutions, measurements of solution concentration, and finally, the qualitative and quantitative evaluation of solution equilibria.