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JEE Main levelSingle correct optionDifficulty 2 of 3qc172

A solution containing $Cu X^{2 +}$ , $Cd X^{2 +}$ , $As X^{3 +}$ and $Bi X^{3 +}$ sulphides (group II precipitate) is warmed with yellow ammonium sulphide. The sulphide that goes into solution is

A $CuS$
B $CdS$
C $As X_{2} S X_{3}$
D $Bi X_{2} S X_{3}$

Answer: C

Group IIB sulphides (As, Sb, Sn) are acidic and form soluble thioanions, e.g.

As X_{2} S X_{3} + 3 S X^{2 -} 2 AsS X_{3} X^{3 -}

. CuS, CdS and

Bi X_{2} S X_{3}

(group IIA) are basic and stay as residue.

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