The subscripts in a chemical formula give relationships between moles of the constituent elements and moles of the compound. Explain why these subscripts do not give relationships between grams of the constituent elements and grams of the compound.
Each element has a different atomic mass number. The grams per mole of one element always differs from the grams per mole of a different element. So, the subscripts that represent mole ratios cannot be used to represent the ratios of grams of a compound.
H2O has 2 moles of hydrogen(H) and 1 mole of oxygen(O) in 1 mole of water. The weight of 1 mole of water is 18 grams. The hydrogen has an atomic weight of approximately 1g and oxygen has approximately 16g. So, the weight of hydrogen per mole of water is 2 grams and oxygen is 16 grams. Here we can see the atomic weight of the atoms is not related to the number of moles of each atom.