When ${\mathrm{H}}_2\mathrm{S}$ gas is passed through an ammonical salt solution X, a slightly white precipitate is formed. The X can be :

When zinc salt solution passes  ${\mathrm{H}}_2\mathrm{S}$ while being in the presence of  ${\mathrm{NH}}_4\mathrm{OH}$ zinc sulphide is formed. ZnS is a white precipitate. 

$\begin{array}{l}{\mathrm{ZnCl}}_2+\mathrm{HCl}\to \mathrm{ZnS}\downarrow \text{ (white ppt) }+2\mathrm{HCl}\\ \left.{\mathrm{ZnCl}}_2+2\mathrm{NaOH}\to \mathrm{Zn}(\mathrm{OH}{)}_2\right)\downarrow \left(\text{ white ppt }\right)+2\mathrm{NaCl}\end{array}$