The Law of Definite Proportions, which states that any chemical compound will always contain a fixed ratio of elements by mass. This law proves that water, from any source, is always made up of two atoms of hydrogen and one atom of oxygen. If we calculate the molecular weight of water, we come up with 18 g/mol. In 1 mole of water, there are 2 grams of hydrogen and 16 grams of oxygen.