Question: how many hydrogen atoms are in 100g of water?
Answer: One molecule of water has 2 atoms of hydrogen and one atom of oxygen. 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.