An aqueous solution of colourless metal sulphate M, gives a white precipitate with $N{H}_{4}OH.$ This was soluble in excess of $N{H}_{4}OH.$ On passing  $H_{2}S$ gas through this solution a white precipitate is formed. The metal M in the salt is :

An aqueous solution of colourless zinc sulphate gives white ppt of $\mathrm{Zn}(\mathrm{OH}{)}_2$ with  ${\mathrm{NH}}_4\mathrm{OH}$

${\mathrm{ZnSOO}}_4+2{\mathrm{NH}}_4\mathrm{OH}\to \mathrm{zn}(\mathrm{OH}{)}_2+{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4$

It was soluble in excess of  ${\mathrm{NH}}_4\mathrm{OH}$ to form Ammonium Zincate ${\left({\mathrm{NH}}_4\right)}_2{\mathrm{ZnO}}_2$

$\mathrm{Zn}(\mathrm{OH}{)}_2+2{\mathrm{NH}}_4\mathrm{OH}\to {\left({\mathrm{NH}}_4\right)}_2{\mathrm{ZnO}}_2+2{\mathrm{H}}_2\mathrm{O}$

On passing $H_{2}S$ through the solution a white ppt of ZnS is formed.

${\left({\mathrm{NH}}_4\right)}_2{\mathrm{ZnO}}_2+{\mathrm{H}}_2\mathrm{S}\to \mathrm{ZnS}+2{\mathrm{NH}}_4\mathrm{OH}$