Question

To precipitate group III cations $N{H}_{4}Cl$ should be added to the solution before the addition of ammonium hydroxide. Explain why.

Answer 1

Solubility products of group (III) hydroxides are low as compared to the hydroxides of higher groups, so a low concentration of OH${}^{-}$ ions is required to precipitate group (III) cations. Presence of $N{H}_{4}Cl$ which is a strong electrolyte suppresses dissociation of weak electrolyte $N{H}_{4}OH$ by common ion effect and decreases OH${}^{-}$ ion concentration considerably.
$N{H}_{4}Cl\rightleftharpoons N{H}_4^{+}+C{l}^{-}$
$N{H}_{4}OH\rightleftharpoons N{H}_4^{+}+O{H}^{-}$