200 moles The balanced equation for creating water from hydrogen and oxygen gas is 2H2 + O2 => 2H2O So for every mole of oxygen gas, you need two moles of hydrogen. So looking that the amount of oxygen and hydrogen you have, it's obvious that oxygen is the limiting reactant since 100 moles of oxygen will consume 200 moles of hydrogen. While 210 moles of hydrogen requires 105 moles of oxygen. Now for each mole of oxygen gas you use, you create 2 moles of water. So 100 mol * 2 = 200 mol So you can create 200 moles of water from the given amounts of reactants.