The Iron (III) ion is 
The sulfate ion is
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.
The sulfate ion is
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.