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So O2 has zero of heat of formation (or enthalpy of formation). The enthalpy of a reaction is defined like this: H = Hproducts-H reactants. The enthalpy of reaction in this case, is the same as the heat of combustion. Hproducts is obtained summing the heats of formation.
Heats of combustion can be calculated from heats of formation DfH. Use PM3 to calculate DfH for octane, butane, butanol, and ethanol. Fill in the first blank column on the following table. Use the NIST Chemistry WebBook to look up literature DfH values for O2, CO2, and H2O.