# Looking back, we see that we have to assume how many atoms of each type are contained in each compound to nd the relative masses of the atoms.

Looking back, we see that we have to assume how many atoms of each type are contained in each compound to nd the relative masses of the atoms. In each of the above examples, we assumed the ratio of atoms to be 1:1 in each compound. If there are atoms of the elements, then this assumption must be wrong, since it gives relative atomic masses which dier from compound to compound. How could we nd the correct atomic ratios? It would help if we knew the ratio of the atomic masses: for example, if we knew that the oxygen to hydrogen mass ratio were 8:1, then we could conclude that the atomic ratio in water would be 1 oxygen and 1 hydrogen. Of the compound (the numbers of atoms of each type), but to know the formula we must know the masses. Which of these possibilities is correct?