Chapter 4 - Energy and Chemical Reactions - Questions for Review and Thought - Topical Questions - Page 189e: 81c

44 kJ/mol.

$\Delta H^{\circ}=\Sigma n_{p}\Delta H_{f}^{\circ}(products)-\Sigma n_{r}\Delta H_{f}^{\circ}(reactants)$ $=[6\Delta H_{f}^{\circ}(HF,g)+\Delta H_{f}^{\circ}(SO_{3},g)]-[\Delta H_{f}^{\circ}(SF_{6},g)+3\Delta H_{f}^{\circ}(H_{2}O,l)]$ $=[6(-271.1\,kJ/mol)+(-395.72\,kJ/mol)]-[(-1209\,kJ/mol)+3(-285.83\,kJ/mol)]$ $=44\,kJ/mol$