How many moles of hydrogen gas would be needed to react with excess carbon dioxide to produce 19.1 moles of water vapor?

Balanced equation:
4H2 + CO2 = 2H2O + CH4

so 4 moles H2 : 2 moles H2O = x : 19.1

you ll need x = 2x19.1 / 4 
and that is 9.55 moles of hydrogen gas