Why `Na_(2)S` cannot be used in place of `H_(2)S` (in presence of `HCl`) as a reagent for `II^(nd)` group cations ?

Explanation

With `Na_(2)S`, along with `2`nd group cations, some cations of higher groups i.e. `III^(rd)` and `IV^(th)` groups may be precipitated because the higher concentration of `S^(2-)` ions is obtained according to following reaction.
`Na_(2)S hArr2 Na^(+)+S^(2-)`
To precipitate `II^(nd)` group cation lower concentration of `S^(2-)` ions is required (because of low `K_(sp)` of `II^(nd)` group sulphides) and this can be obtained easily by `H_(2)S` in presence of dilute `HCl`.
`H_(2)S hArr2 H^(+)+S^(2-),HCl hArrH^(+)+Cl^(-)`
Due to common ion effect the ionisation of `H_(2)S` is suppressed and the concentration of `S^(2-)` ions obtained is just sufficient to precipitate the cations of `II^(nd)` group.