$\mathrm{MgSO}_{4}$ on reaction with $\mathrm{NH}_{4} \mathrm{OH}$ and $\mathrm{Na}_{2} \mathrm{HPO}_{4}$ forms a white crystalline precipitate. What is its formula?

(A) $\mathrm{Mg}(\mathrm{NH}_{4}) \mathrm{PO}_{4}$

(B) $\mathrm{Mg}_{3}(\mathrm{PO}_{4})_{2}$

(C) $\mathrm{MgCl}_{2} . \mathrm{MgSO}_{4}$

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

MgSO_{4}

on reaction with

NH_{4} OH

and

Na_{2} HPO_{4}

forms a white crystalline precipitate. What is its formula?

(A) $Mg (NH_{4}) PO_{4}$ ✓ Correct
(B) $Mg_{3} (PO_{4})_{2}$
(C) $MgCl_{2} . MgSO_{4}$
(D) $MgSO_{4}$

Answer: (A)

Solution

This is a qualitative test for magnesium ions. When

Mg^{2 +}

reacts with ammonium hydroxide and disodium hydrogen phosphate, it forms magnesium ammonium phosphate as a white crystalline precipitate:

Mg^{2 +} + NH_{4} OH + Na_{2} HPO_{4} \to Mg (NH_{4}) PO_{4} ↓ + H_{2} O + Na^{+}

The precipitate has the formula

MgNH_{4} PO_{4}

(or

Mg (NH_{4}) PO_{4}

JEE Advanced · 2006 · 09 Apr 2006 · One or more correct options

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