Final answer:
The conjugate acid-base pair among the options is HCN and CN-, where HCN is the proton donor and CN- is the proton acceptor. The correct option is 2.
Step-by-step explanation:
The conjugate acid-base pair concept is part of the Brønsted-Lowry theory of acids and bases. To identify a pair, we look for two species that differ by the presence of one hydrogen ion (H+). A conjugate acid-base pair consists of a proton donor (acid) and a proton acceptor (base).
In the choices provided, the pair that represents a conjugate acid-base pair is HCN and CN-. HCN can donate a proton to become CN-, thereby making them a conjugate acid-base pair.
On the other hand, CH3COOH and OH- do not form a conjugate acid-base pair because OH- is not the base form of acetic acid; HCN and OH- are not a pair because they do not directly convert into each other by the gain or loss of a proton; and HCl and OH- are not a conjugate acid-base pair as they represent an acid and a base from two different pairs, not one directly related by donation or acceptance of a single proton.