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# $NH_4OH$ is a weak base but it becomes still weaker in the aqueous solution of

(a) 0.1 M $HCl$

(b) 0.1 M $NH_4Cl$

(c) 0.1 M $H_2SO_4$

(d) 0.1 M $CH_3COOH$

Toolbox:
• Common-ion effect describes the suppressing effect on ionization of an electrolyte when when another electrolyte is added that shares a common ion.
Answer: 0.1 M $NH_4Cl$

Adding a common ion prevents the weak acid or weak base from ionizing as much as it would without the added common ion. The common ion effect suppresses the ionization of a weak acid by adding more of an ion that is a product of this equilibrium.

Hence, Degree of Ionisation of $NH_4OH$ is suppressed by $NH_4Cl$ due to common ion effect.