Solid NH4Cl is added before NH4OH in the group III analysis mainly because it

suppresses the ionisation of NH4OH (common ion effect), keeping [OH-] low so that only the least soluble hydroxides precipitate

increases the [OH-] so that all hydroxides precipitate

suppresses the ionisation of NH4OH (common ion effect), keeping [OH-] low so that only the least soluble hydroxides precipitate

forms soluble ammine complexes with Fe^3+ and Al^3+

NEET levelSingle correct optionDifficulty 1 of 3qc003

Solid $NH X_{4} Cl$ is added before $NH X_{4} OH$ in the group III analysis mainly because it

Aincreases the $[OH X^{-}]$ so that all hydroxides precipitate
Bsuppresses the ionisation of $NH X_{4} OH$ (common ion effect), keeping $[OH X^{-}]$ low so that only the least soluble hydroxides precipitate
Cforms soluble ammine complexes with $Fe X^{3 +}$ and $Al X^{3 +}$
Doxidises $Fe X^{2 +}$ to $Fe X^{3 +}$

NH X_{4} OH

is a weak base and

NH X_{4} Cl

supplies a large

[NH X_{4} X^{+}]

, which pushes the ionisation equilibrium back and keeps

[OH X^{-}]

small. At this low

[OH X^{-}]

only

Fe (OH) X_{3}

Al (OH) X_{3}

and

Cr (OH) X_{3}

exceed their

K_{s p}

, while

Mg (OH) X_{2}

and

Mn (OH) X_{2}

stay dissolved.