Question

The pH of aqueous ammonium chloride solution is:
A. less than 7
B. equal to 7
C. greater than 7
D. greater than or equal to 7

Answer 1

Hint: The concept behind this question is not to write the experimental value of pH of ammonium chloride but to check whether this salt comes under weak salt or strong salt which in turn can be summarized by looking at the bases and acids it is made up of. If strong acids and bases are its constituents then, strong salt and if either acid or base is weak, then, it is considered as weak salt.

Complete answer:
Ammonium chloride is a salt which has the chemical formula $\text{N}{{\text{H}}_{4}}\text{Cl}$. To check whether it is weak salt or strong salt. Let us look at the acid and bases it is made up of and the reaction.
Ammonium chloride is made up of ammonia $\left( \text{N}{{\text{H}}_{3}} \right)$ and hydrochloric acid $\left( \text{HCl} \right)$, the reaction is $\text{N}{{\text{H}}_{3}}+\text{HCl}\to \text{N}{{\text{H}}_{4}}\text{Cl}$. We know that ammonia $\left( \text{N}{{\text{H}}_{3}} \right)$ is a weak base because it does not contains hydroxide ions, but on reaction with water, it produces hydroxide ions but the reaction is almost reversible; $\text{N}{{\text{H}}_{3}}+{{\text{H}}_{2}}\text{O}\rightleftharpoons \text{NH}_{4}^{+}+\text{O}{{\text{H}}^{-}}$. Moreover, the electronegativity of nitrogen atoms makes it less basic, as nitrogen atoms take hold on its lone pair electrons to leave its vicinity. Hydrochloric acid is a strong mineral acid as it readily dissociates to produce hydrogen ions $\left( {{\text{H}}^{+}} \right)$ and chloride ions $\left( \text{C}{{\text{l}}^{-}} \right)$ . The reaction is $\text{HCl}\to {{\text{H}}^{+}}+\text{C}{{\text{l}}^{-}}$.
So, it is clear that ammonium chloride is made of weak base and strong acid. Ammonium chloride is considered as a neutral salt but actually its solution is slightly acidic because chloride ion $\left( \text{C}{{\text{l}}^{-}} \right)$ is weak conjugate base of strong acid $\left( \text{HCl} \right)$ and ammonium ion $\left( \text{NH}_{4}^{+} \right)$ is a strong conjugate acid of a weak base $\left( \text{N}{{\text{H}}_{3}} \right)$. Thus, in the solution the conjugate acid is $\left( \text{NH}_{4}^{+} \right)$ stronger than the weak base $\left( \text{C}{{\text{l}}^{-}} \right)$ ; so, the solution is slightly acidic.
Acidic solution means the pH is less than 7 by the relation $\text{pH =}-\text{log}\left[ {{\text{H}}^{+}} \right]$.
The pH of aqueous ammonium chloride solution is less than 7. The correct answer is option ‘a’.

Note:
When a salt is composed of strong acid and strong base, then, its pH is exactly equal to 7. Like, when hydrochloric acid reacts with sodium hydroxide to sodium chloride and water. The pH of sodium chloride is 7. The reaction is $\text{HCl}+ \text{NaOH} \to \ \text{NaCl}+\text H_{2}{\mathrm O}$.
Moreover, strong base + weak acid gives basic salt and strong acid + weak base gives acidic salt.