Ferric sulphate on heating gives :

Anhydrous ferrous sulphate, ${\mathrm{FeSO}}_4$, is produced when ferrous sulphate crystals are heated and lose their water content. As a result, they turn from bright green to white. Anhydrous ferrous sulphate breaks down into ferric oxide (${\mathrm{Fe}}_2{\mathrm{O}}_3$), sulphur dioxide (${\mathrm{SO}}_2$), and sulphur trioxide with additional heating (${\mathrm{SO}}_3$).

${\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3⟶{\mathrm{Fe}}_2{\mathrm{O}}_3+3{\mathrm{SO}}_3$.