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In III group precipitation, $\mathrm{NH}_4 \mathrm{Cl}$ is added before adding $\mathrm{NH}_4 \mathrm{OH}$ to
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The correct answer is:
decrease conc. of $\mathrm{OH}^{-}$
In III group precipitation, $\mathrm{NH}_4 \mathrm{Cl}$ is added before adding $\mathrm{NH}_4 \mathrm{OH}$ to decrease the conc. of$\mathrm{OH}^{-}$, as $\mathrm{NH}_4 \mathrm{Cl}$ and $\mathrm{NH}_4 \mathrm{OH}$ dissociates as follows.
$\begin{aligned}
& \mathrm{NH}_4 \mathrm{Cl} \rightleftharpoons \mathrm{NH}_4^{+}+\mathrm{Cl}^{-} \\
& \mathrm{NH}_4 \mathrm{OH} \rightleftharpoons \underset{\substack{\text { common } \\
\text { ion }}}{\mathrm{NH}_4^{+}+\mathrm{OH}^{-}}
\end{aligned}$
Due to presence of common ion, conc. of $\mathrm{OH}^{-}$ decreases and for III group precipitation lesser conc. of $\mathrm{OH}^{-}$is required.
$\begin{aligned}
& \mathrm{NH}_4 \mathrm{Cl} \rightleftharpoons \mathrm{NH}_4^{+}+\mathrm{Cl}^{-} \\
& \mathrm{NH}_4 \mathrm{OH} \rightleftharpoons \underset{\substack{\text { common } \\
\text { ion }}}{\mathrm{NH}_4^{+}+\mathrm{OH}^{-}}
\end{aligned}$
Due to presence of common ion, conc. of $\mathrm{OH}^{-}$ decreases and for III group precipitation lesser conc. of $\mathrm{OH}^{-}$is required.
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