Answer
-98.9 kJ
Work Step by Step
$\Delta H^{o}_{rxn}=\Sigma n_{p}\Delta H_{f}^{o}(products)-\Sigma n_{r}\Delta H_{f}^{o}(reactants)$
$=[1(\Delta H^{o}_{f,\,SO_{3}(g)})]-[1(\Delta H^{o}_{f,\,SO_{2}(g)})+\frac{1}{2}(\Delta H^{o}_{f,\,O_{2}(g)})]$
$=1(-395.7\,kJ)-[1(-296.8\,kJ)+\frac{1}{2}(0)]$
$=-98.9\,kJ$
