Advanced Placement (AP) Chemistry Practice Exam

Which of the following is true about weak acids?

1. They dissociate completely in solution.

2. Most of the acid molecules remain in solution.

3. They always have a high pKa value.

4. They only dissociate at high temperatures.

The correct answer is: Most of the acid molecules remain in solution.

The statement that most of the acid molecules remain in solution is true for weak acids. Unlike strong acids, which fully dissociate into ions when dissolved in water, weak acids only partially dissociate. This partial dissociation means that a significant proportion of the weak acid molecules remain intact in solution. As a result, the equilibrium of dissociation lies towards the reactants, indicating that the concentration of undissociated acid is higher compared to the dissociated ions. Understanding the behavior of weak acids involves recognizing how their dissociation is characterized quantitatively by the acid dissociation constant (Ka). A smaller Ka indicates a weaker acid, which corresponds to a higher concentration of undissociated acid in the solution. In contrast, the process of complete dissociation is a hallmark of strong acids, making the first statement incorrect. The relationship between pKa and acid strength means that while weak acids often have higher pKa values than strong acids, not all weak acids have high pKa values, as this is dependent on their specific strength. The concept that weak acids only dissociate at high temperatures is not generally true, as they can dissociate at various temperatures; temperature impacts the equilibrium but does not solely determine dissociation.