Question: how many grams of hydrogen and oxygen will you need?
Answer: 1 mole of water requires 1 mole of hydrogen (H2) and 0.5 moles of oxygen (O2). This requires 100/18 moles of hydrogen. As the molar mass of hydrogen is 2, it requires 200/18 = 100/9 g of hydrogen. It requires 100/9 g of hydrogen gas and 800/9 g of oxygen gas to make 100 g of water.