Question

Ammonium ion is:
(A) neither an acid or base
(B) both an acid and a base
(C) a conjugate acid
(D) a conjugate base

Answer 1

Hint: Ammonium ion is formed when ammonia gets protonated (gain of ${{H}^{+}}$ion) when reacted with hydrochloric acid. Here ammonia is a weak base and hydrochloric acid is a strong acid. Gain and loss of proton by acid and base form conjugate acid –base pair.

Complete step by step solution:
As you know from your chemistry lessons that acid has the tendency to donate proton and base has the tendency to accept proton (Bronsted lowry concept).
-The absence and presence of protons in acid and base form conjugate acid-base pairs.
-Here, ammonia $\left( N{{H}_{3}} \right)$ a weak base gets protonated or accept a proton from hydrochloric acid $\left( HCl \right)$ which a strong acid to form ammonium ion.
\[N{{H}_{3}}\,+\,HCl\to \,NH_{4}^{+}\,+C{{l}^{-}}\]
\[N{{H}_{3}}\,+\,{{H}^{+}}C{{l}^{-}}\to \,NH_{4}^{+}\,+\,C{{l}^{-}}\]
-In this reaction ammonium ion behaves as a conjugate acid of $N{{H}_{3}}$and conjugate acids are formed when acid donates its proton basically ${{H}^{+}}$ to the base. Thus $C{{l}^{-}}$ is a conjugate base of $HCl$ .Conjugate bases are from when a proton is removed by an acid.
-From the reaction you have seen that ammonium ions have a positive charge on it this is due to the addition of one extra ${{H}^{+}}$in the ammonia.
-Ammonia is a weak base react with hydrochloric acid a strong acid to form ammonium ion which is a conjugate weak acid.

Thus, option (C) is the correct one.

Note: Do not confuse between the Arrhenius and Bronsted lowry concept. Conjugate acid-base pair theory was given by Bronsted lowry. Here, we usually talk about the gain and loss of protons and not about the electrons (Arrhenius concept).