Q5 / 56 · KVPY 2018 (SA) · Chemistry

Among the following, the correct statement is

Correct answer
A
$p H$ decreases when solid ammonium chloride is added to a dilute aqueous solution of $N H_{3}$
B
$p H$ decreases when solid sodium acetate is added to a dilute aqueous solution of acetic acid
C
$p H$ decreases when solid $N a C l$ is added to a dilute aqueous solution of $N a O H$
D
$p H$ decreases when solid sodium oxalate is added to a dilute aqueous solution of oxalic acid

Solution

Dil.aqueous solution of $N H_{3}$ is $N H_{4} O H$

$N H_{4} O H ⇌ N H_{4}^{+} + O H^{-}$

On adding solid ammonium chloride

$N H_{4} C l ⟶ N H_{4}^{+} + C l^{-}$

The reaction moves backward due to common ion effect. The concentration of $O H^{-}$ decreases and hence the $p H$ decreases.

Hence, the correct answer is option: 'A'.