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JEE Advance - Chemistry (2013 - Paper 2 Offline - No. 12)

An aqueous solution of a mixture of two inorganic salts, when treated with dilute HCl, gave a precipitate (P) and a filtrate (Q). The precipitate (P) was found to dissolve in hot water. The filtrate (Q) remained unchanged, when treated with H2S in a dilute mineral acid medium. However, it gave a precipitate (R) with H2S in an ammoniacal medium. The precipitate R gave a coloured solution (S), when treated with H2O2 in an aqueous NaOH medium.
An aqueous solution of a mixture of two inorganic salts, when treated with dilute HCl, gave a precipitate (P) and a filtrate (Q). The precipitate (P) was found to dissolve in hot water. The filtrate (Q) remained unchanged, when treated with H2S in a dilute mineral acid medium. However, it gave a precipitate (R) with H2S in an ammoniacal medium. The precipitate R gave a coloured solution (S), when treated with H2O2 in an aqueous NaOH medium.
An aqueous solution of a mixture of two inorganic salts, when treated with dilute HCl, gave a precipitate (P) and a filtrate (Q). The precipitate (P) was found to dissolve in hot water. The filtrate (Q) remained unchanged, when treated with H2S in a dilute mineral acid medium. However, it gave a precipitate (R) with H2S in an ammoniacal medium. The precipitate R gave a coloured solution (S), when treated with H2O2 in an aqueous NaOH medium.
The coloured solution S contains
$$F{e_2}{(S{O_4})_3}$$
$$CuS{O_4}$$
$$ZnS{O_4}$$
$$N{a_2}Cr{O_4}$$

Explanation

The reactions involved are as follows:

$$\mathop {C{r^{3 + }}}\limits_{(Q)} + 3N{H_4}OH\buildrel {{H_2}S} \over \longrightarrow \mathop {Cr{{(OH)}_3}}\limits_{(R)\,(Green\,ppt.)} + 3NH_4^ + $$

$$\mathop {2Cr{{(OH)}_3}}\limits_{(R)} + 3{H_2}{O_2} + 4NaOH \to \mathop {2N{a_2}Cr{O_4} + 8{H_2}O}\limits_{(S)\,(Yellow\,solution)} $$

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