The concept of cmmon ion effect applies to the dissociation of weak electrolytes. Their dissociation gets further suppressed in the presence of common ions furnished by a suitable electrolyte. Similarly the solubility of sparingly soluble salts in water gets suppresessed by the presence of a common ions. You are provided with an aqueous solution containing `Cu^(2+) " and " Zn^(2+)` ions.
(i) How are these ions precipitated from the solution
(ii) What is the colour of the precipiate formed ?
(iii) How does dilute HCI help in checking the precipitation of `Zn^(2+)` ions in group II?
(iv) How does `NH_(4)OH` help in promoting the precipitation of `Zn^(2+)` ions in group IV?`

Explanation

(i) These ions are precipitated as their sulphides (Cus and ZnS) on passing `H_(2)S` through the aqueous solution of the mixture
(ii) CuS is back in colour while ZnS has a white colour.
(iii) Dilute HCI helps in controlling the `S^(2-)` ion concentration in solution due to common ion effect `(H^(+)` ions are common ion) . Only CuS is precipitated since its `K_(sp)` is less. ZnS remains in solution as its `K_(sp)` as its is more.
`H_(2)S overset((aq))(to) 2H^(+) (aq) + S^(2-) (aq)`
`HCI overset((aq)) (to) H^(+) (aq) +CI^(-) (aq)`
(iv) `NH_(4) OH` helps in increasing `S^(2-)` ion concentration in aqueous solution.
`H_(2)S overset((aq)) (hArr) 2H^(+) (aq) +S^(2-) (aq)`
`NH_(4)OH overset((aq))(hArr) NH_(4)^(+) (aq) + OH^(+) (aq)`
`H^(+) (aq) + OH^(-) (aq) hArr underset(("Weakly dissociated"))(H_(2)O (aq))`
As result more of `S^(2-)` ions are released and the concentration of `Zn^(2+)` ions and `S^(2-)` ions in aqueous solution exceed the `K_(sp)` value of ZnS. Therefore it gets precipiated in aqueous solution.