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.
$$N{H}_3+HCl\to N{H}_4^{+}+C{l}^{-}$$
$$N{H}_3+H^{+}C{l}^{-}\to N{H}_4^{+}+C{l}^{-}$$
-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).