The chemical formula for the compound Iron (III) sulphide is:

The chemical formula for the compound Iron (III) sulphide is ${\mathit{Fe}}_2{S}_3$.

Iron (Fe) has a valency of +3 and nitrate ($S^{2-}$) has a valency of -2.
Therefore, by criss-cross method, we can find the formula of the compound as :
${\mathit{Fe}}^{3+}$       $S^{2-}$
 

2                  3
Therefore, the formula of the compound becomes ${\mathit{Fe}}_2{S}_3$.