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JEE Main levelSingle correct optionDifficulty 2 of 3qa060

Magnesia mixture ( $MgCl X_{2}$ + $NH X_{4} Cl$ + aqueous $NH X_{3}$ ) is added to a solution of sodium hydrogen phosphate. The white crystalline precipitate formed is

A $MgHPO X_{4}$
B $Mg (OH) X_{2}$
C $MgNH X_{4} PO X_{4}$
D $Mg X_{3} (PO X_{4}) X_{2}$

Answer: C

In ammoniacal solution phosphate,

Mg X^{2 +}

and

NH X_{4} X^{+}

combine to give the double salt

MgNH X_{4} PO X_{4} \cdot 6 H X_{2} O

. The

NH X_{4} Cl

in the reagent suppresses

OH X^{-}

so that

Mg (OH) X_{2}

does not separate. Hence the precipitate is

MgNH X_{4} PO X_{4}

.

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