The Iron (III) ion is \(Fe^{3+}\)
The sulfate ion is \(SO _{4}^{2-}\)
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.
