how many grams of water can be produced from 10 moles of hydrogen gas?
Since you have 10 moles of hydrogen, at that ratio you will need 5 moles of oxygen to for the reaction to use all the hydrogen. Since oxygen weighs ~16 grams per mole, you add 5 × 16 g, or 80 g to the 10.1 g from the 10 moles of hydrogen to get 90.1 grams of H2O.