Advanced Placement (AP) Chemistry Practice Exam

Which of the following best describes a conjugate acid-base pair?

1. They are always both strong acids.

2. They differ by one proton (H+).

3. They have identical chemical formulas.

4. They consist of two bases.

The correct answer is: They differ by one proton (H+).

A conjugate acid-base pair is defined by the relationship between an acid and its corresponding base, which differ by exactly one proton (H+). When an acid donates a proton, what remains is its conjugate base. Conversely, when a base accepts a proton, it becomes its conjugate acid. This clear distinction based on the presence or absence of one proton is what defines the pair. The other choices do not accurately represent the characteristics of conjugate acid-base pairs. For instance, not all conjugate pairs consist of strong acids; they can be weak acids and bases as well. Identical chemical formulas cannot describe a conjugate pair, as they must differ by that single proton. Finally, a conjugate acid-base pair cannot consist solely of two bases, as this would negate the essential role of proton transfer that characterizes acid-base chemistry. Thus, the hallmark feature is the difference of one proton, making the choice accurate.