If 9 g of water contains 1 g of hydrogen and 8 g of oxygen, then 36 g of water will contain:

This calculation is based on the Law of Constant Proportions, which states that a chemical compound always contains its component elements in a fixed ratio by mass, regardless of the source or total amount. In this specific case, the constant mass ratio of hydrogen to oxygen in water is 1:8, therefore, for any total mass of water, hydrogen accounts for 1/9 and oxygen accounts for 8/9 of that mass. For a 36 g sample, the amount of hydrogen is 36 x (1/9) = 4 gm and the amount of oxygen is 36 x (8/9) = 32 gm, maintaining the exact 1:8 proportion.