Question: how many moles of hydrogen gas are produced from 5.00 mol of water?
Answer: From this you can see that 5 moles of hydrogen gas would react with 2.5 of the available 3 moles of oxygen gas, to form 5 moles of water. The molar mass of water is 18 g/mol, so you would end up with 5 x 18 = 90 g of water.