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JEE Main levelSingle correct optionDifficulty 2 of 3qc014

The filtrate from group II is boiled before the group III reagent is added. The main purpose is to

Aexpel $NH X_{3}$ from the filtrate
Bprecipitate $Pb X^{2 +}$ as $PbCl X_{2}$
Cexpel dissolved $H X_{2} S$ , which would otherwise give sulphides of group IV cations in the ammoniacal medium
Dconvert $Fe X^{3 +}$ to $Fe X^{2 +}$

Answer: C

Adding

NH X_{4} OH

to a solution still containing

H X_{2} S

raises

[S X^{2 -}]

and would precipitate

ZnS

,

NiS

,

CoS

and

MnS

together with the group III hydroxides. Boiling removes

H X_{2} S

; no

NH X_{3}

is present yet, and

Fe X^{2 +}

is oxidised, not

Fe X^{3 +}

reduced, in the next step.

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