Calculate the number of moles of the reacting substance by dividing the number of grams of the substance by the molar mass of the compound. A mole is a chemical measurement equivalent to a number of molecules of the compound. Of course, because every compound has a different individual weight, a mole of different substances has a different mass. Methanol, for instance, has a molar mass of 32 grams per mole, so if you had 16 grams of methanol you would have half a mole of methanol (32/16=.5).
Obtain the theoretical enthalpy change from combustion for the reactant from a chemistry reference. Returning to the example of methanol, the compound has an enthalpy change of 728 Kilojoules (Kj) per mole, meaning that every mole of the compound releases 728 Kj of heat when combusted.
Multiply the number of moles of the substance by the enthalpy of combustion. In the case of the combustion of half a mole of methanol, the enthalpy change would be 364 Kj (728 Kj/mole x .5 moles). Hess's law states that every molecule of a substance releases the same amount of heat upon combustion, so that the combustion of half a mole of methanol will release half the heat expected from the combustion of a full mole.