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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.
The idea here is that the standard molar enthalpy of combustion, Î´Hâcomb, tells you the enthalpy change associated with the combustion of one mole of a given substance under standard conditions. This tells you that every mole of methane that undergoes combustion under standard conditions gives off 890.8 kJ of heat. All you have to do here is use the molar mass of methane to see how many moles you have in that 50.00 g sample. 50.00gâ1 mole CH416.0425g=3.1167 moles CH4.