What best describes a conjugate acid-base pair?

• A. A pair of substances that differ by a proton
• B. A pair of substances that differ by an electron
• C. A pair of substances that differ only by a proton
• D. A pair of substances that have identical conjugate partners

Answer: (C)

In Brønsted-Lowry acid-base chemistry, a conjugate acid-base pair is two species that differ by exactly one proton. When an acid donates a proton, it becomes its conjugate base; when a base accepts a proton, it becomes its conjugate acid. The defining idea is that the two forms are identical in every way except for that single proton.

That’s why the best description is a pair that differs only by a proton. For example, ammonia and ammonium differ by one proton, as do acetic acid and acetate. These pairs illustrate how proton transfer links an acid to its conjugate base (or a base to its conjugate acid).

Options suggesting differences in electrons describe redox changes, which aren’t about proton transfer. The idea of identical conjugate partners doesn’t make sense because conjugate forms are distinct species linked by gaining or losing a proton.