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For the Hess' Law calculation:
Eqs #1 to 3 are enthalpies of formation. The enthalpy of any reaction is given by:
DH = DHf (products) - DHf (reactants). Remember to halve the DH value for SO3, because the one given is for formation of 2 moles of SO3.
I get an answer of between -400 ...
The answers are given regarding the Hess Law/Oxidation with short explanations.