Kaplan MCAT General Chemistry Review

The relative acidity or basicity of an aqueous solution is determined by the relative concentrations of acid and base equivalents. An acid equivalent is equal to one mole of H⁺ (or H₃O⁺) ions; a base equivalent is equal to one mole of OH^- ions. Some acids and bases are polyvalent; that is, each mole of the acid or base liberates more than one acid or base equivalent. For example, the divalent acid H₂SO₄ undergoes the following dissociation in water:

H₂SO₄ (aq) + H₂O (l) H₃O⁺ (aq) + HSO₄^- (aq)

HSO₄^- (aq) + H₂O (l) H₃O⁺ (aq) + SO₄^2- (aq)

One mole of H₂SO₄ can produce two acid equivalents (2 moles of H₃O⁺). You’ll notice, if you look closely at the dissociation reaction for sulfuric acid, the first dissociation goes to completion but the second dissociation goes to an equilibrium state. The acidity or basicity of a solution depends upon the concentration of acidic or basic equivalents that can be liberated. The quantity of acidic or basic capacity is directly indicated by the solution’s normality (see Chapter 9, Solutions). For example, since each mole of H₃PO₄ can yield three moles (equivalents) of H₃O⁺, a 2 M H₃PO₄ solution would be 6 N (6 normal).

Another measurement useful for acid-base chemistry is equivalent weight. Chapter 4 defined and discussed this term extensively, and Chapter 9 reviewed it briefly. The gram equivalent weight is the mass of a compound that produces one equivalent (one mole of charge). For example, H₂SO₄ (molar mass: 98 g/mol) is a divalent acid, so each mole of the acid compound yields two acid equivalents. The gram equivalent weight is 98/2 = 49 grams. That is, the complete dissociation of 49 grams of H₂SO₄ will yield one acid equivalent (one mole of H₃O⁺). Common polyvalent acids include H₂SO₄, H₃PO₄, and H₂CO₃. Common polyvalent bases include CaCO₃, Ca(OH)₂, and Mg(OH)₂. Magnesium hydroxide is the active ingredient in the thick, “milky” over-the-counter preparation that is used as an antidote for occasional acid indigestion and diarrhea. Calcium carbonate (CaCO₃) is the active ingredient in the over-the-counter tablet preparation also used to treat an upset tummy.

Amphoteric Species

An amphoteric, or amphiprotic, species is one that reacts like an acid in a basic environment and like a base in an acidic environment. In the Brønsted-Lowry sense, an amphoteric species can either gain or lose a proton. Water is the most common example. When water reacts with a base, it behaves as an acid:

H₂O + B^- HB + OH^-

When water reacts with an acid, it behaves as a base:

HA + H₂O H₃O⁺ + A^-

The partially dissociated conjugate base of a polyvalent acid is usually amphoteric (e.g., HSO₄^- can either gain an H⁺ to form H₂SO₄ or lose an H⁺ to form SO ₄^2-). The hydroxides of certain metals (e.g., Al, Zn, Pb, and Cr) are also amphoteric. Furthermore, species that can act as either oxidizing or reducing agents (see Chapter 11, Redox Reactions and Electrochemistry) are considered to be amphoteric as well, because by accepting or donating electron pairs, they act as Lewis acids or bases, respectively. A simple way of remembering the meaning of the term is to think of amphibian animals, such as frogs. Amphibians live both on land and in water. Water is the most common amphoteric species, being able to act as both an acid and a base, and amphibians (both land and water living) = amphoteric compounds (both acid and base). As you might suspect, this is no mere linguistic coincidence; amphi comes from the Greek prefix meaning “on both sides” or “of both kinds.”

Bridge

Recall that we spoke about gram equivalent weights in Chapter 4 and about normality and all units of concentration in Chapter 9.

Titration and Buffers

Titration is a procedure used to determine the molarity of a known reactant in a solution. There are different types of titrations, including redox, acid-base, and complexometric (metal ion). The MCAT will likely test your understanding of titration procedures and calculations in the context of acid-base solutions. (Metal ion titration may appear on Test Day, but it is less commonly tested and will likely have an informative passage attached.) Titration is a common laboratory procedure, and almost certainly you have performed at least one titration (probably acid-base) in your academic career.