Which compound is formed as a white crystalline precipitate when Na2HPO4 is added to a solution of Mg^2+ containing NH4Cl and NH4OH?

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Which compound is formed as a white crystalline precipitate when $Na X_{2} HPO X_{4}$ is added to a solution of $Mg X^{2 +}$ containing $NH X_{4} Cl$ and $NH X_{4} OH$ ?

In the presence of

NH X_{4} X^{+}

and

NH X_{3}

the phosphate precipitates magnesium as magnesium ammonium phosphate:

Mg X^{2 +} + NH X_{3} + HPO X_{4} X^{2 -} Mg (NH X_{4}) PO X_{4}

. This is the standard confirmatory test for group VI.