The chemical formula for iron sulfate depends on the oxidation state of the iron ion:

* Iron(II) sulfate (ferrous sulfate) is FeSO₄

* Iron(III) sulfate (ferric sulfate) is Fe₂(SO₄)₃

Remember that "ferrous" refers to iron in its +2 oxidation state, and "ferric" refers to iron in its +3 oxidation state.