We are given with the equation that produces sulfuric acid from the reaction of sulfur trioxide gas and water. This is expressed in the balanced equation: SO3+ H2O= H2SO4. The enthalpy of the reaction in an aqueous solution can be determined by taking the difference between the summation of enthalpies of the products multiplied to their respective stoichiometric coefficient and the summation of enthalpies of the reactants multiplied to their respective stoichiometric coefficient. Answer is -689.9-(-368-237) equal to -84.9 kJ per mole.