Write formulas for the following compounds:
$(1)$ Iron $(III)$ sulphate
$(2)$ Chromium $(III)$ oxide

(N/A) For Iron $(III)$ sulphate:
Iron has an oxidation state of $+3$ and the sulphate ion is $SO_{4}^{2-}$. By criss-crossing the valencies,we get $Fe_{2}(SO_{4})_{3}$.
For Chromium $(III)$ oxide:
Chromium has an oxidation state of $+3$ and the oxide ion is $O^{2-}$. By criss-crossing the valencies,we get $Cr_{2}O_{3}$.