How many liters of H2 are required to react completely with 5.7 liters of O2?

Answer 1

2H2 + O2 ==> 2H2O
When all gases are involved you can take a shortcut and NOT calculate mols. Use L directly.
5.7 L O2 x (2 mols H2/1 mol O2) = 5.7 x 2/1 = ?