In the precipitation of the iron group (III) | vedclass

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(B) The dissociation of ammonium hydroxide is given by: $NH_{4}OH \rightleftharpoons NH_{4}^{+} + OH^{-}$.Ammonium chloride is a strong electrolyte an

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decrease concentration of ${OH}^{-}$ ions

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(B) The dissociation of ammonium hydroxide is given by: $NH_{4}OH \rightleftharpoons NH_{4}^{+} + OH^{-}$.Ammonium chloride is a strong electrolyte and dissociates completely: $NH_{4}Cl \rightarrow NH_{4}^{+} + Cl^{-}$.Due to the common ion effect of $NH_{4}^{+}$,the equilibrium of $NH_{4}OH$ shifts to the left.This decreases the concentration of $OH^{-}$ ions to such an extent that only the hydroxides of group-$III$ cations (like $Fe^{3+}$,$Al^{3+}$,$Cr^{3+}$) are precipitated,as they have very low $K_{sp}$ values (in the range of $10^{-18}$ to $10^{-38}$),while other group cations remain in solution.

In the precipitation of the iron group $(III)$ in qualitative analysis,ammonium chloride is added before adding ammonium hydroxide to :

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