Advanced Placement (AP) Chemistry Practice Exam

What characterizes a buffer solution?

1. It is a solution that can only resist changes in pH at low concentrations.

2. It can either be acidic or basic but not neutral.

3. It consists of a weak acid and its conjugate base or a weak base and its conjugate acid.

4. It requires the presence of a strong acid to function effectively.

The correct answer is: It consists of a weak acid and its conjugate base or a weak base and its conjugate acid.

A buffer solution is characterized by the ability to maintain a relatively stable pH when small amounts of acid or base are added to it. This stability arises from the presence of a weak acid paired with its conjugate base or a weak base with its conjugate acid. The components in a buffer solution work together to neutralize added acids or bases, thereby minimizing shifts in pH. For instance, in an acidic buffer, the weak acid can donate protons (H⁺) when a base is added, while the conjugate base can accept protons when acid is introduced. This dual capability allows the buffer system to effectively resist changes in pH across a relevant range, providing significant control over the solution’s acidity or basicity. In contrast to the other options, a buffer does not have to exclusively be acidic or basic, nor does it only operate within low concentrations; it can function across various concentrations. Additionally, a buffer does not need the presence of a strong acid for its effectiveness, as its primary mechanism relies on the weak acid-base pairs rather than strong acids or bases. This distinctive composition is what makes option C the defining characteristic of a buffer solution.