Chemistry - enthalpy of combustion(help!!!)

i need help with the below question, i have tried answering it, but do not know where to go after the equations, please start me off on how to find the enthalpy of combustion of at least one....

Consider methane, CH4, and hydrogen, H2, as possible fuel sources.
(a) Write the chemical equation for the complete combustion of each fuel. Then find the enthalpy of combustion, ΔHcomb, of each fuel. Express your answers in kJ/mol and kJ/g. Assume that water vapour, rather than liquid water, is formed in both reactions.

CH4 + 2O2 ↔ 2H2O + CO2
2H2 + O2 ↔ 2H2O

should i use the enthalpy of formations and its equations??

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  1. i tried using it for CH4, below is my working am i correct? cause i researched a website(cannot post link)
    and it uses enthalpy of formation!!!
    i am using the molar enthalpy of formation equation:
    (ndeltaHf products) - (ndeltaHf reactants)

    CH4 + 2O2 ↔ 2H2O + CO2
    deltaCOMB = [(2*-241.8)+(-393.5)]-[-74.6]
    =-502.5kJ

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  2. Yes, that's the way to go with the CH4. My set of tables gave -74.81 kJ/mol for CH4 but use what you have in your tables. Just check them to make sure the numbers are right. The -502.5 kJ (I didn't check the math) is kJ/mol. To convert to grams you multiply by molar mass CH4.
    For H2 use
    2H2 + O2 ==> 2H2O. Remember H2 and O2 will be zero since they are in their "standard" state.

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