The ionic compound containing Fe3+ and (SO4)2− would have the formula
Fe2(SO4)3
The valency of Fe3+ ion is 3 and that of (SO4)2− ion is 2. So, on cross-multiplication of valencies, the formula of the ionic compound containing Fe3+ and (SO4)2− is Fe2(SO4)3. It is also called as iron(III) sulphate or Ferric sulphate.