2H2 + O2 = 2H2O you look for the mole ratio according to the balanced chemical equation. 1 mole of oxygen reacts with 2 mole of hydrogen to produce 2 mole of of H2O, the ratio is 1:2 x mole of oxygen : 2x mole of H2O : x=13.35 mole 13.35 mole of oxygen : 2*13.35 mole of H2O 26.70 moles of water produced