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NEET levelSingle correct optionDifficulty 1 of 3qc061

In the group III analysis, $NH X_{4} Cl$ is added before $NH X_{4} OH$ mainly to

Araise $[OH X^{-}]$ so that all hydroxides precipitate
Bconvert $Al X^{3 +}$ into the soluble chloro complex
Coxidise $Fe X^{2 +}$ to $Fe X^{3 +}$
Dsuppress the ionisation of $NH X_{4} OH$ and so lower $[OH X^{-}]$

Answer: D

The common ion

NH X_{4} X^{+}

pushes the equilibrium

NH X_{4} OH NH X_{4} X^{+} + OH X^{-}

back, giving a low

[OH X^{-}]

. This is enough to exceed the very small

K_{s p}

of group III hydroxides but not the larger

K_{s p}

of

Mg (OH) X_{2}

or

Mn (OH) X_{2}

.

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