A mixture of group IV sulphides ( $ZnS$ , $MnS$ , $NiS$ , $CoS$ ) is treated with cold dilute $HCl$ . The residue is dissolved in aqua regia, the solution is made ammoniacal and a reagent is added. Which sulphides remain in the residue, and which reagent gives a rose-red precipitate for one of the metals?

Answer: C

ZnS

and

MnS

dissolve in cold dilute HCl, but freshly precipitated

NiS

and

CoS

turn into less soluble forms and remain undissolved. Aqua regia dissolves them, and in ammoniacal solution

Ni X^{2 +}

gives the rose-red

Ni (DMG) X_{2}