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In the systematic precipitation of Group III basic radicals (), the prior addition of solid before iQualitative and Quantitative Analysis Chemistry Question

Question

In the systematic precipitation of Group III basic radicals ($Fe^{3+}, Al^{3+}, Cr^{3+}$), the prior addition of solid $NH_4Cl$ before $NH_4OH$ is theoretically mandatory in order to:

Answer: B

💡 Solution & Explanation

$NH_4Cl$ provides a high concentration of $NH_4^+$ ions, shifting the ionization equilibrium of $NH_4OH$ backwards (common ion effect). The resulting low $[OH^-]$ is sufficient to precipitate the very low $K_{sp}$ Group III hydroxides but insufficient to precipitate Group IV-VI hydroxides.

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