The Iron (III) ion is [tex]Fe^{3+} [/tex]
The sulfate ion is [tex]SO _{4}^{2-} [/tex]
Thus when forming the compound iron (iii) sulfate you would need three sulfate ions bonded to two iron ions in order to fill the valency requirement
∴ the formula would be Fe₂(SO₄)₃
Whenever you are adding a number to a radical you must bracket it.
