1. The problem statement, all variables and given/known data The sun has a mass of 2*10^30kg and gives off energy at a rate of 3.9*10^26 watts. If the source of the sun's energy energy were ordinary combustion of methane how long will it last. 2. Relevant equations delta Enthalpy of the combustion of methane is -802 kj. Energy equals watts times time. 3. The attempt at a solution All I need to know is this. When I do this problem do I use the mass of both the methane and 2O_2 which is 80 grams per mole or just the methane which is 16 grams per mole. I did this problem using 80 grams per mole because that's the mass of the fuel that is burning that gives off 802 kj of energy.