Advanced Placement (AP) Chemistry Practice Exam

Which of the following describes polyprotic acids?

1. They donate only one proton.

2. They can donate multiple protons.

3. They contain no oxygen.

4. They are always strong acids.

The correct answer is: They can donate multiple protons.

Polyprotic acids are defined by their ability to donate more than one proton (H⁺ ion) per molecule in a stepwise manner. This characteristic distinguishes them from monoprotic acids, which only donate a single proton. Examples of polyprotic acids include sulfuric acid (H₂SO₄) and phosphoric acid (H₃PO₄), which can undergo multiple ionization steps, each resulting in the release of a proton. The process of donating multiple protons typically occurs in a series of dissociation reactions. For instance, sulfuric acid first donates one proton to form the bisulfate ion (HSO₄⁻) and can subsequently donate another proton to form sulfate (SO₄²⁻). Each step has its own equilibrium constant, indicating the strength of the acid at that stage. The other options do not accurately describe the nature of polyprotic acids. They do not only donate one proton, as clearly established. Polyprotic acids can, in fact, contain oxygen (like most common acids). Lastly, being polyprotic does not intrinsically relate to the strength of the acid; there are both strong polyprotic acids, such as sulfuric acid, and weak ones