what is the heat of combustion of methanol
The sign of the heat of formation matters. Using hess's law: 2(-238kJ) + 4(242kJ) + 2(393)= -1279=Heat of reaction Heat of combustion of methanol is just the heat of reaction divided by the molar coefficient for methanol in the chemical equation so about -639.5kJ/mol. In acuality it is somewhere near -727kJ/mol.