Kaplan MCAT General Chemistry Review

The solubility of substances into different solvents is ultimately a function of thermodynamics. When the change in Gibbs function is negative at a given temperature for the dissolution of a given solute into a given solvent, the process will be spontaneous, and the solute is said to be soluble. When the change in Gibbs function is positive at a given temperature for the dissolution of a given solute into a given solvent, the process will be nonspontaneous, and the solute is said to be insoluble. Some solute/solvent systems have very large negative Gs, so dissolution is very spontaneous and a lot of solute can be dissolved into the solvent. Others have very small negative Gs, so dissolution is only slightly spontaneous and as a result only a little solute can be dissolved into the solvent. Those solutes that dissolve minimally in the solvent (usually water) are called the sparingly soluble salts.

AQUEOUS SOLUTIONS

The most common type of solution is the aqueous solution, in which the solvent is water. The aqueous state is denoted by the symbol (aq). Because aqueous solutions are so common and so important to biological systems (e.g., you), the MCAT focuses on them above all others. We wish we could tell you otherwise, but for Test Day, you are expected to remember the general solubility rules for aqueous solutions. We know, we know—they are not the easiest or most fun to remember, but no one ever promised you that this would always be fun. (We promised you that we would help you have some fun in this preparation process.) There are seven general solubility rules:

1. All salts of alkali metals are water soluble.

2. All salts of the ammonium ion (NH₄⁺) are water soluble.

3. All chlorides, bromides, and iodides are water soluble, with the exceptions of Ag⁺, Pb²⁺, and Hg₂²⁺.

4. All salts of the sulfate ion (SO₄^2-) are water soluble, with the exceptions of Ca²⁺, Sr²⁺, Ba²⁺, and Pb²⁺.

5. All metal oxides are insoluble, with the exception of the alkali metals and CaO, SrO, and BaO, all of which hydrolyze to form solutions of the corresponding metal hydroxides.

6. All hydroxides are insoluble, with the exception of the alkali metals and Ca ²⁺, Sr²⁺, and Ba²⁺.

7. All carbonates (CO₃^2-), phosphates (PO₄^3-), sulfides (S^2-), and sulfites (SO₃^2-) are insoluble, with the exception of the alkali metals and ammonium.

On the MCAT, there is one infallible solubility rule: All sodium salts are completely soluble, and all nitrate salts are completely soluble. Thus, if a problem gives you a concentration of sodium fluoride, you know that the compound is completely soluble in water. Sodium and nitrate ions are generally used as counterions to what is really chemically important; for example, if a pH problem gives you a sodium formate concentration of 0.10 M, it is really telling you that the concentration of the formate ion is 0.10 M, because the sodium ion concentration does not affect pH. The only time you need to worry about the nitrate ion concentration is in a redox reaction, for the nitrate ion can function—though only weakly—as an oxidizing agent; otherwise, merely focus on the cation as the chemically reacting species.

MCAT Expertise

Because most solutions involve water as the solvent in the real world, it is not a surprise that they are common on the MCAT. These solubility rules are not bad to know, but memorizing them all may be a little excessive. It is never a bad thing to know facts, but being able to apply them is more important. Know rules 1 and 2 for sure and be aware of some of the more common insoluble exceptions, like Pb²⁺ and Ag⁺.

Ions

Ionic solutions are of particular interest to chemists because certain important types of chemical reactions—acid-base and oxidation-reduction reactions, for instance—take place in ionic solutions. It shouldn’t come as a surprise to you by now that if chemists take particular interest in this area of chemistry, so will the MCAT. In fact, acid-base and oxidation-reduction reactions are themselves important topics for the MCAT, so you can begin your review of those topics (Chapters 10 and 11) by reviewing with us now the characteristics and behaviors of ions.

CATIONS AND ANIONS