Determine how many moles of water vapor will be produced at 1.77 atm and 200°C by the complete combustion of 1

If you could explain how to do it, that would be awesome. I'm doing a web assign.

combustion reaction: CH4 + 2O2 --> CO2 + 2H2O.
if u have 18.9 L methane gas, use PV=nRT.
So 1.77*18.9=n*.0821*473.15.
n is number of moles, so n=.861 moles of CH4. Molar ratio is 1 CH4: 2 H2O,
so you will have .861*2 moles water= 1.72 moles h2o