Question

Which one of the following salts on dissolution in water will give a solution of pH less than 7?

• A. $\text{NaCl}$
• B. ${\text{CH}}_3\text{COONa}$
• C. ${\text{NH}}_4\text{Cl}$
• D. $\text{KCN}$

Answer 1

The correct option is C ${\text{NH}}_4\text{Cl}$

The reaction in which an acid and a base reacts to form the corresponding salt and water is called neutralisation reaction. The table given below shows the pH value of the given salts and the reason behind their pH value.
$$\begin{array}{cccc}\text{Salt}& \text{Combination}& \text{Type of Combination}& \text{pH Value}\\ \text{KCN}& \text{HCN + KOH}& \text{Weak acid+Strong base}& \text{>7}\\ {\text{CH}}_3\text{COONa}& {\text{CH}}_3\text{COOH + NaOH}& \text{Weak acid+Strong base}& \text{>7}\\ \text{NaCl}& \text{HCl + NaOH}& \text{Strong acid+Strong base}& \text{=7}\\ {\text{NH}}_4\text{Cl}& {\text{HCl + NH}}_4\text{OH}& \text{Strong acid+Weak base}& \text{<7}\end{array}$$It is clear from the table that among the given options only ammonium chloride $\left({\text{NH}}_4\text{Cl}\right)$ has pH value less than seven and hence it is an acidic salt.