First slide

Salt hydrolysis :

Question

Hydrolysis constant of salt derived from strong acid and weak base is 2 × 10^–5. The dissociation constant of the weak base is

Moderate

A

5 x10^-8
B

5 x 10^-9
C

5 x 10^-10
D

2 x 10^-19

Solution

NH₄Cl is a salt of strong acid and weak base.
${NH}_{4} Cl \overset{H_{2} O}{⟶} {NH}_{4}^{+} (aq) + {Cl}^{-} (aq)$
${NH}_{4}^{+}$ (aq) will not undergo hydrolysis
${NH}_{4}^{+} + H_{2} O ⇌ {NH}_{4} OH + H_{3} O^{+}$
$K_{h} = {(K_{a})}_{{NH}_{4}^{+}} = \frac{K_{w}}{{(K_{b})}_{{NH}_{4} OH}}$
$\Rightarrow 2 \times 10^{- 5} = \frac{10^{- 14}}{{(K_{b})}_{{NH}_{4} OH}}$
$\Rightarrow {(K_{b})}_{{NH}_{4} OH} = 5 \times 10^{- 10}$

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