1. The acid-base pair that differs only by one proton is called conjugate acid-base pair. Such acid-base pairs are formed by loss or gain of a proton.
2. Consider the ionization of hydrochloric acid in water.
HCl(aq) acts as an acid by donating a proton to H2O molecule which acts as a base because it accepts the proton. The species H2O+ is produced when water accepts a proton from HCl. Therefore, Cl- is the conjugate base of HCl and HCl is the conjugate acid of Cl- . Similarly, H2O is the conjugate base of H2O+ and H3O++ is the conjugate acid of the base H2O.
