Advanced Placement (AP) Chemistry Practice Exam

A buffer is a solution that resists changes in pH when small amounts of an acid or base are added. This typically involves a weak acid and its conjugate base or a weak base and its conjugate acid.

In the case of acetic acid and sodium acetate, acetic acid serves as a weak acid, while sodium acetate provides the conjugate base (acetate ion). Together, they create a solution that can effectively maintain a stable pH upon the addition of acids or bases, fulfilling the criteria for a buffer system.

The other pairs lack the necessary components for buffering. Strong acids like HCl do not contain a weak acid or its conjugate base, leaving no mechanism to resist pH changes. NaOH is a strong base, and H2O does not contribute to a buffering system. Lastly, carbon dioxide and oxygen do not represent an acid-base pair that can stabilize pH, as they are not involved in a conjugate acid-base relationship. Thus, the pair of acetic acid and sodium acetate is the only correct option that satisfies the conditions of a buffer solution.