Question

Which one of the following salts on being dissolved in water gives pH$ > 7$ at $25^0C$?
A.$KCN$
B.$KN{O_3}$
C.$N{H_4}Cl$
D.$N{H_4}CN$

Answer 1

Hint: First we will check whether the salt is of strong base and weak acid or not. If on dissolving it gives a strong base and weak acid then only it will show basic character. The pH will be determined on the basis of the character of the salt. If the salt is basic in nature then it will give pH greater than $7$ and if the salt is acidic in nature then pH will be less than$7$.

Complete step by step answer:
When potassium cyanide $KCN$is dissolved in water then it forms potassium hydroxide and hydrogen cyanide.
The reaction is given as-$KCN + {H_2}O \rightleftharpoons KOH + HCN$
Here potassium hydroxide is a strong base and hydrogen cyanide is a weak acid so we can say that $KCN$ is salt of strong base and weak acid.
 Due to the presence of a strong base, this solution will give the pH$ > 7$ at $25^0C$ $KN{O_3}$ is a salt of potassium hydroxide and nitric acid. We know that $KOH$ is a strong base and $HN{O_3}$ is a strong acid so it will not hydrolyze and the pH will remain$7$.
$N{H_4}Cl$ forms $N{H_4}OH$ and $HCl$ when it is dissolved in water. Ammonium hydroxide is a weak base and hydrochloric acid is a strong acid. So the salt will have acidic character and will give pH less than$7$.
$N{H_4}CN$ is salt of weak acid and weak base. It gives $N{H_4}OH$ and $HCN$ on being dissolved in water. So it will have acidic character as the relative strength of acid is more than base so it will give pH less than $7$.

The correct answer is option A.

Note:
The salt of a weak acid and weak base can be neutral, acidic or basic as its p[H depends on the relative strength of the weak base and weak acid which is formed on hydrolysis of the salt. Let ${K_a}$ be the dissociation constant of weak acid and ${K_b}$ be the dissociation constant of weak base then-
If${K_a} > {K_b}$ , then the solution is acidic.
If${K_a} = {K_b}$ , then the solution is neutral.
If${K_a} < {K_b}$ , then the solution is basic.