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Q.
$NH_4OH$ is a weak base but it becomes still weakerin the aqueous solution of
Equilibrium
Solution:
Common ion effect is observed when a solution of weak electrolyte is mixed with a solution of strong electrolyte which provides an ion common to that provided by a weak electrolyte.
Ammonium hydroxide is a weak electrolyte and ammonium chloride is a strong electrolyte. Ammonium chloride provides ammonium ion which is common to that provided by ammonium hydroxide.
Thus the pair $NH _{4} OH + NH _{4} Cl$ shows common ion effect. Ammonium chloride suppresses the ionization of ammonium hydroxide.
Hence, ammonium hydroxide becomes still weaker in the presence of an aqueous solution of ammonium chloride.