Using the standard heats of formation below, what is the heat of reaction when sulfur dioxide reacts with oxygen to form sulfur trioxide? 2so2(g) + o2(g) → 2so3(g) ∆hf° so2(g) = –298.8 kj/mol ∆hf° so3(g) = –395.7 kj/mol –193.8 kj –201.9 kj –492.6 kj –694.5 kj

To calculate the heat of reaction we need to subtract the summ of all reagent's formation heat from the summ of all product's formation heat.
For this example it will be:
2*Hf(SO3) - ( 2*Hf(SO2) + Hf(O2) ) =
2 * (-395.7) - ( 2 * (-298.8) + 1 * 0) =
-791.4 + 597.6 = - 193.8