The group II filtrate (containing Fe^3+ reduced to Fe^2+, Zn^2+ and Mg^2+) is treated with NH4Cl and NH4OH without boiling off the dissolved H2S. The solid obtained is most likely

a mixture of black FeS and white ZnS, with Mg^2+ remaining in solution

JEE Advanced levelSingle correct optionDifficulty 3 of 3qc043

The group II filtrate (containing $Fe X^{3 +}$ reduced to $Fe X^{2 +}$ , $Zn X^{2 +}$ and $Mg X^{2 +}$ ) is treated with $NH X_{4} Cl$ and $NH X_{4} OH$ without boiling off the dissolved $H X_{2} S$ . The solid obtained is most likely

Aa mixture of black $FeS$ and white $ZnS$ , with $Mg X^{2 +}$ remaining in solution
B $FeS$ , $ZnS$ and $MgS$
Conly white $ZnS$
Donly $Fe (OH) X_{3}$

Ammonia neutralises the acid and makes the medium basic while

H X_{2} S

is still present, so

[S X^{2 -}]

rises sharply and both

FeS

(black) and

ZnS

(white) precipitate prematurely.

Fe X^{2 +}

is not oxidised without

HNO X_{3}

, so no

Fe (OH) X_{3}

forms.

MgS

is hydrolysed and not precipitated, so

Mg X^{2 +}

stays dissolved.