Chapter 16 - Thermodynamics: Directionality of Chemical Reactions - Questions for Review and Thought - Topical Questions - Page 737d: 66

-305 kJ/mol

$\Delta_{r}G^{\circ}=\Sigma n_{p}\Delta_{f}G^{\circ}(products)-\Sigma n_{r}\Delta _{f}G^{\circ}(reactants)$ $\implies -37.2\,kJ/mol=[\Delta_{f}G^{\circ}(PCl_{5},g)]-[\Delta_{f}G^{\circ}(PCl_{3},g)+\Delta_{f}G^{\circ}(Cl_{2},g)]$ $-37.2\,kJ/mol=[\Delta_{f}G^{\circ}(PCl_{5},g)]-[(-267.8\,kJ/mol)+(0)]$ $\implies \Delta_{f}G^{\circ}(PCl_{5},g)=-37.2\,kJ/mol-267.8\,kJ/mol$ $=-305\,kJ/mol$