A solution when diluted with $\mathrm{H}_{2} \mathrm{O}$ and boiled, it gives a white precipitate. On addition of excess $\mathrm{NH}_{4} \mathrm{Cl} / \mathrm{NH}_{4} \mathrm{OH}$. the volume of precipitate decreases leaving behind a white gelatinous precipitate. Identify the precipitate which dissolves in $\mathrm{NH}_{4} \mathrm{OH} / \mathrm{NH}_{4} \mathrm{Cl}$.

(A) $\mathrm{Zn}(\mathrm{OH})_{2}$

(B) $\mathrm{Al}(\mathrm{OH})_{3}$

(C) $\mathrm{Mg}(\mathrm{OH})_{2}$

(D) $\mathrm{Ca}(\mathrm{OH})_{2}$

JEE Advanced · 2006 · 09 Apr 2006 · Practical and Analytical Chemistry

A solution when diluted with

H_{2} O

and boiled, it gives a white precipitate. On addition of excess

NH_{4} Cl / NH_{4} OH

. the volume of precipitate decreases leaving behind a white gelatinous precipitate. Identify the precipitate which dissolves in

NH_{4} OH / NH_{4} Cl

(A) $Zn (OH)_{2}$ ✓ Correct
(B) $Al (OH)_{3}$
(C) $Mg (OH)_{2}$
(D) $Ca (OH)_{2}$

Answer: (A)

Solution

The precipitate that dissolves in

NH_{4} OH / NH_{4} Cl

Zn (OH)_{2}

. Zinc hydroxide is amphoteric and forms a soluble tetraammine zinc(II) complex:

Zn (OH)_{2} + 4 NH_{3} \to [Zn (NH_{3})_{4}]^{2 +} + 2 OH^{-}

. The white gelatinous precipitate that remains is

Al (OH)_{3}

, which does not dissolve in ammonia.

JEE Advanced · 2006 · 09 Apr 2006 · Single correct option

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