Conjugate acid-base : The base formed by the donation of a single proton by an acid is called the conjugate base of the acid. Similarly, the acid formed by the acceptation of a single proton by a base is called the conjugate acid of the base.
Conjugate acid-base pair : The acid and the conjugate base, that are related to each other by donating and accepting a single proton are said to constitute a conjugate acid-base pair.
HCl + H2O ↔ Cl― + H3O+
HCl + H2O ↔ Cl― + H3O+
From the above reaction, we can see that, acid HCl gives up its proton and produced a new base Cl―. Thus Cl― is the conjugate base of the acid HCl, and HCl and Cl― is a conjugate acid-base pair. Similarly the original base H2O accepted the proton and produced the acid H3O+, which is conjugate acid. Thus H3O+ and H2O is a conjugate acid-base pair.
An original strong acid or base has a respectively weak conjugate base or acid and vice versa. That can be shown by the following reaction,
