You first need to write your chemical equation:
CO2 + 4 H2 → CH4 + 2 H2O (balanced equation)
4 moles of hydrogen gas will produce 2 moles of water vapor since carbon dioxide is in excess.
ratio is then 4 H2 / 2 H2O
If you produce 30.6 moles of water vapor, how many moles of hydrogen gas is needed:
let's solve with ratio 4 H2 / 2 H2O = x H2 / 30.6 H2O
Therefore you need twice as many moles of hydrogen gas: 61.2 moles of hydrogen gas is needed.
Hope it helps.
