Question

The pH of which salt solution is independent of its concentration
$(1)$ $(C{H}_{3}COO)C_5{H}_{5}NH$
$(2)$ $Na{H}_{2}P{O}_4$
$(3)$ $N{a}_{2}HP{O}_4$
$(4)$ $N{H}_{4}CN$

• A. 1, 2, 3, 4
• B. 1, 4
• C. 2, 3
• D. 1, 2, 3

Answer 1

Answer: (A)

pH of amphoteric salts and weak acid weak base salt is independent of its concentration as they act like buffer.
The $pH$ Of the salts formed by weak acids and weak bases does not depend on the concentration of the salt.
$pH=7+\frac{1}{2}(pKa-pKb)$
$\left({\left(CH\right)}_{3}COO\right)C_6{H}_{5}NH$ and $N{H}_{4}CN$ are salts of a weak acid and weak base.
The salt $\left({\left(CH\right)}_{3}COO\right)C_6{H}_{5}NH$ is formed from weak acid acetic acid and weak base aniline.
The salt $N{H}_{4}CN$ is formed from $weakbaseN{H}_{4}OHandweakacidHCN$
The salts $Na{H}_{2}P{O}_{4}andN{a}_{2}HP{O}_4$ are formed from a weak acid $H_{3}P{O}_4$ and stong base $NaOH$ .
Both are amphoteric salts. Their $pH$ is the average of respective $p{K}_a$ values.
ex: $pH=\frac{pKa1+pKa2}{2}$
Hence, their $pH$ is also independent of the concentration of the salts.