NH4Cl and NH4OH must be present when (NH4)2CO3 is added in group V, mainly to

avoid precipitation of Mg^2+ as carbonate and keep CO3^2- available (not HCO3-) for group V

precipitate Mg^2+ along with Ca^2+

JEE Main levelSingle correct optionDifficulty 2 of 3qc020

$NH X_{4} Cl$ and $NH X_{4} OH$ must be present when $(NH X_{4}) X_{2} CO X_{3}$ is added in group V, mainly to

Adissolve $BaCO X_{3}$ and $SrCO X_{3}$
Bavoid precipitation of $Mg X^{2 +}$ as carbonate and keep $CO X_{3} X^{2 -}$ available (not $HCO X_{3} X^{-}$ ) for group V
Coxidise $Mn X^{2 +}$ to $MnO X_{2}$
Dprecipitate $Mg X^{2 +}$ along with $Ca X^{2 +}$

The ammonium ion/ammonia buffer keeps

[CO X_{3} X^{2 -}]

at a controlled level, so

Mg X^{2 +}

does not precipitate as carbonate, and

CO X_{3} X^{2 -}

is not converted to

HCO X_{3} X^{-}

by hydrolysis.

BaCO X_{3}

SrCO X_{3}

CaCO X_{3}

precipitate normally.