JEE Main · 2024 · 04 Apr 2024 · Shift 1 · Practical and Analytical Chemistry

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

(A)prevent interference by phosphate ions
(B)decrease concentration of $^{-} OH$ ions✓ Correct
(C)increase concentration of $Cl^{-}$ ions
(D)increase concentration of $NH_{4}^{+}$ ions

Answer: (B)

Solution

N H_{4} Cl

supplies a high

N H_{4}^{+}

concentration. By the common-ion effect this suppresses the ionization of

N H_{4} OH

, **lowering

[O H^{-}]

**. The reduced

[O H^{-}]

keeps group-IV cations in solution while still precipitating the group-III hydroxides, which have very low

K_{s p}

. Option B.

JEE Main · 2024 · 04 Apr 2024 · Shift 1 · Single correct option

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