Answer:
In group III, the cations \[F{{e}^{3+}},\,C{{r}^{3+}}\] and \[A{{l}^{3+}}\] are precipitated as hydroxides by the addition of \[N{{H}_{4}}OH\] to the aqueous solution of the mixture. But a small amount of \[N{{H}_{4}}Cl\] (common \[NH_{4}^{+}\] ion) is added before the addition of \[N{{H}_{4}}OH\]. Its purpose is to suppress the \[NH_{4}^{+}\] ion concentration in solution due to common ion effect.
Thus, only the hydroxides of cations of group III (low \[{{K}_{sp}}\] values) will be precipitated while the hydroxides of cations of higher groups will remain in solution since their \[{{K}_{sp}}\] values are higher and the products will remain less than their solubility product \[({{K}_{sp}})\] values.
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