The enthalpy of formation of magnesium oxide MgO cannot be determined using the given information. The enthalpies of formation provided are for magnesium Mg and oxygen O2 , which are the reactants in the formation of magnesium oxide. To find the enthalpy of formation of MgO, you would need the enthalpy change for the reaction:Mg s + 1/2 O2 g MgO s This enthalpy change, also known as the heat of formation, is specific to the formation of one mole of magnesium oxide from its constituent elements in their standard states. If this value is provided, then you can determine the enthalpy of formation of MgO.