Answer & Explanation
A conjugate acid, is a species formed by the reception of a proton (H+) by a base—in other words, it is a base with a hydrogen ion added to it.
On the other hand, a conjugate base is what is left over after an acid has donated a proton during a chemical reaction. Hence, a conjugate base is a species formed by the removal of a proton from an acid.
Because some acids are capable of releasing multiple protons, the conjugate base of an acid may itself be acidic.
Represented as the following chemical reaction:
Acid + Base ⇌ Conjugate Base + Conjugate Acid
The water molecule is the conjugate acid of the hydroxide ion after the latter received the hydrogen proton donated by ammonium. On the other hand, ammonia is the conjugate base for the acid ammonium after ammonium has donated a hydrogen ion towards the production of the water molecule. We can also refer to OH- as a conjugate base of H2O, since the water molecule donates a proton towards the production of NH+4 in the reverse reaction, which is the predominating process in nature due to the strength of the base NH3 over the hydroxide ion.