[diah]

If you're given a reaction, such as;
2O(g)-->O2(g) with delta H=-249 kJ/mol OR
C(graphite)+2H2(g)-->CH4(g) with delta H=-75 kJ/mol

then how do you calculate the heat of combustion of methane into gaseous H2O?

I found the enthalpies of H2O(g) which is -241.8 kJ/mol and CH4 is -74.9 kJ/mol, do you use the equation of delta H=(sum of products)-(sum of reactants) or something else?

[Corribus]

Always start with a balanced chemical reaction equation.  Which in your case would be.... what do you think?

[diah]

Would you add in O2 and CH4 on the reactant side and then H2O(g) on the product side?

[Corribus]

You're missing something.  And also, you need to balance the equation.

[diah]

I got 4O(g)+CH4(g)-->2H2O(g)+CO2(g)

What is missing?

[Corribus]

You must be careful. Methane doesn't react with "O(g)".