Indentify the correct order of solubility in aqueous medium

Options

A. `ZnS gt Na_(2)S gt CuS`
B. `Na_(2)S gt CuS gt ZnS`
C. `Na_(2)S gt ZnS gt CuS`
D. `CuS gt ZnS gt Na_(2)S`

Answer

3

Explanation

Alkali metal sulphides are all water soluble and hydrolyse appreciably in water ,giving strognly alkaline solutions:
`Na_(2)S+H_(2)OrarrNaSH+NAOH`
`ZnS `is more soluble than `Cus` and because `ZnS` is precipitated in alkaline medium while `CuS` is precipitated in acidic medium
` H_(2)S(aq.) hArr 2H^(+)(aq.)+S^(2-)(aq.)`
Thus we need smaller conc of `S^(2-)` ions to precipitate `CuS` whilst we required higher conc. of `S^(2-)` ions to precipitate `ZnS`. This question can be also be credited to chemical equilibrium and salt analysis