I'm reviewing for a test and stumbled upon this question:

What mass of natural gas (CH4) must you burn to emit 261 kJ of heat?

CH4 + 2O2 --> CO2 + 2H2O

(delta) H rxn = -802.3 kJ

I know it involves stoichiometry, but I'm stumped on how to solve it! Any help would be great thanks!

Ok, first, you know that one mole of CH4 (methane or natural gas) gives off 802.3 kJ when completely burnt. Work out the number of moles for 261 kJ, it's simple!

802.3 kJ -> 1 mol
261 kJ -> \frac{261}{802.3} moles.

One mol of CH4 has a mass of (12+1+1+1+1) = 16g

Work out for the number of moles you obtained before.

Hope it helped:)!