The reaction 2 "Mg"(s) + "O"_2(g) -> 2 "MgO"(s) has a free energy DeltaG_"rxn" = -"597 kJ/mol" under standard conditions. What is the free energy of formation of "MgO"?

The given reaction provides the Gibbs free energy of formation for the formation of two moles of MgO. Therefore, the ${G}_{f}$ for one mole is half that amount.