Advanced Placement (AP) Chemistry Practice Exam

Which statement about weak acids is correct?

1. They completely ionize in solution.

2. They exist mainly in undissociated form in solution.

3. They are pH neutral in all concentrations.

4. They contain no H+ ions in solution.

The correct answer is: They exist mainly in undissociated form in solution.

Weak acids are characterized by their incomplete ionization in solution, which is a key aspect of their behavior in water. When a weak acid is dissolved, it establishes an equilibrium between the undissociated acid molecules and the ions produced from their partial ionization. This means that, in a solution of a weak acid, a significant fraction of the acid remains in the original form rather than fully dissociating into ions. Consequently, statement describing weak acids as mainly existing in undissociated form accurately captures this fundamental property. The other statements do not reflect the nature of weak acids. Weak acids do not completely ionize in solution, which discredits the first statement. They can certainly have pH values that are not neutral depending on their concentration and the specific acid, so the assertion about being pH neutral in all concentrations is also incorrect. Furthermore, weak acids do contain H+ ions in solution, as they release some hydrogen ions upon ionization, contradicting the last statement. Therefore, the correct statement about weak acids emphasizes their predominance in undissociated form when in solution.