Hess's Law is as follows. Note: Google Hess's law for the actual formula (i.e. with the Greek characters).
ΔH(reaction) = Sum of ΔH(formation of products) - Sum of ΔH(formation of reactants)
Now determine you're formula/reaction. In this case it's as follows:
CH4 + 2 O2 -----> CO2 + 2 H2O
The ΔH(formation) can be found in standardized tables, either online, in books, or provided to you. The ΔH you need for this is as follows in kJ/mol:
CH4 (g) -75
O2 (g) 0
CO2 (g) -394
H2O (l) -286
When you plug the info into the formula you get the following:
ΔH(reaction) = [(2*-238) + -394)] - [-75 + (2*0)]
ΔH(reaction) = -891 kJ/mol
Now you're looking for the amount of kJ for 1.65 mol. Your standard formula is as follows:
E = ΔH(reaction) * mol
where E is energy measured in kJ.
So when you plug in your relevant values you get:
E = -891 * 1.65
E = -1470.15 kJ
