SAT Subject Test Chemistry
REVIEW OF MAJOR TOPICS
Acids, Bases, and Salts
Buffer solutions are equilibrium systems that resist changes in acidity and maintain constant pH when acids or bases are added to them. A typical laboratory buffer can be prepared by mixing equal molar quantities of a weak acid such as HC2H3O2 and its salt, NaC2H3O2. When a small amount of a strong base such as NaOH is added to the buffer, the acetic acid reacts (and consumes) most of the excess OH− ion. The OH− ion reacts with the H+ ion from the acetic acid, thus reducing the H+ ion concentration in this equilibrium:
This reduction of H+ causes a shift to the right, forming additional C2H3O2− ions and H+ ions. For practical purposes, each mole of OH− added consumes 1 mole of HC2H3O2 and produces 1 mole of C2H3O2− ions.
When a strong acid such as HCl is added to the buffer, the H+ ions react with the C2H3O2− ions of the salt and form more undissociated HC2H3O2. This does not alter the H+ ion concentration. Proportional increases and decreases in the concentrations of C2H3O2− and HC2H3O2 do not significantly affect the acidity of the solution.