Given the thermochemical equation 2SO2 + O2 → 2SO3, ΔH°rxn = -198 kJ/mol, what is the standard enthalpy change for the decomposition of one mole of SO3?

Answer 1

You want 1 mol SO3 and not 2 so divide by 2. You want the reverse so change the sign.

Answer 2

99 kJ/mol