Kaplan MCAT General Chemistry Review

Solutions are homogenous mixtures of two or more substances that combine to form a single phase, generally the liquid phase. The most important kind of solution for the MCAT is the aqueous solution. Solvents dissolve solutes by a process of surrounding the solute particles and interacting with them by way of electrostatic forces; this is called solvation.

Most dissolutions are endothermic. However, the dissolution of gas into liquid is exothermic.

Solubility is the maximum amount of substance that can be dissolved in a particular solvent at a particular temperature. Molar solubility is the molar concentration of solute in a saturated solution.

Units of solution concentration include percent composition by mass (mass of solute divided by mass of solution times 100%), mole fraction (moles of solute divided by total number of moles of substances in solution), molarity (moles of solute divided by liters of solution), molality (moles of solute divided by kilograms of solvent), and normality (number of equivalents divided by liters of solution).

A saturated solution is in equilibrium for that particular temperature. K_sp is the solubility product constant for a given solute in a given solvent at a given temperature.

Calculation of the ion product (I.P.), followed by comparison to the known K_sp, helps to determine if a solution is unsaturated (I.P < K_sp), saturated (I.P = K_sp), or supersaturated (I.P. > K_sp).

When an ionic compound is dissolved into a solution that already contains one of the constituent ions, the molar solubility for that ionic compound will be significantly decreased from the value normally demonstrated by the same ionic compound in the pure solvent at the same temperature. This is the common ion effect. It is an application of Le Châtelier’s principle to solutions.

EQUATIONS TO REMEMBER

M_iV_i = M_fV_f (the dilution law)

K_sp = [Aⁿ+]^m[B^m-]ⁿ for A_mB_n (s) mAⁿ+ (aq) + nB^m- (aq)

I.P. = [Aⁿ⁺]^m[B^m-]ⁿ

Practice Questions

1. An aqueous solution was prepared by mixing 70 grams of sugar (C₁₂H₁₂O₁₁) into 100 grams of water. The solution has a boiling point of 101.11°C. What is the molar mass of the solute? (K_b = 0.512°C.)

A. 322.58 g/mol

B. 32.26 g/mol

C. 123.24 g/mol

D. 233.59 g/mol

2. Which phase of solvent and solute, respectively, can form a solution?

I. Solid solvent, gaseous solute

II. Solid solvent, solid solute

III. Gaseous solvent, gaseous solute

A. I and II only

B. II and III only

C. I and III only

D. I, II, and III

3. Two organic liquids, pictured in the figure below, are combined to form a solution. Based on the structures, will the solution closely obey Raoult’s law?

A. Yes, the liquids differ due to the additional methyl group on toluene and, therefore, will not deviate from Raoult’s law.

B. Yes, the liquids are very similar and, therefore, will not deviate from Raoult’s law.

C. No, the liquids differ due to the additional methyl group on toluene and, therefore, will deviate from Raoult’s law.

D. No, the liquids both contain benzene rings, which will interact with each other and cause deviation from Raoult’s law.

4. The diagram in Figure 1 shows two arms separated by an impermeable membrane. What would happen to the level of liquid in the two branches if the membrane were replaced with a semipermeable membrane that allowed water molecules to move across the membrane?

Figure 1

A. The level on the right would decrease, and the level on the left would increase.

B. The level would remain the same on both sides.

C. The level on the right would increase, and the level on the left would decrease.

D. The level on the right would stay the same, and the level on the left would increase.

5. The process of formation of a liquid solution can be better understood by breaking the process into three steps:

1. Breaking the solution into individual components

2. Making room for the solute in the solvent by overcoming intermolecular forces in the solvent

3. Allowing solute–solvent interactions to occur to form the solution

The overall energy change to form a solution can be estimated by taking the sum of each of the three steps. For steps 1 through 3, will each step most likely be endothermic or exothermic? The order for each answer choice is step 1, step 2, followed by step 3.

A. Endothermic, exothermic, endothermic

B. Exothermic, endothermic, endothermic

C. Exothermic, exothermic, endothermic

D. Endothermic, endothermic, exothermic

6. The entropy change when a solution forms can be expressed by the term S°_soln. When an ion dissolves and water molecules are ordered around it, the ordering would be expected to make a negative contribution to S°_soln. An ion that has more charge density will have a greater hydration effect, or ordering of water molecules. Based on this information, which of the following compounds will have the most negative S°_soln?

A. KCl

B. LiF

C. CaS

D. NaCl