What is the percent composition of a compound that contains 40.8 g carbon and 54.4 g oxygen?

Percent composition tells you which types of atoms (elements) are present in a molecule and their levels. Percent composition can also tell you about the different elements present in an ionic compound as well.

Let's look at two examples

The molar mass of #H_2O# is 18 g/mol The hydrogens make up 2g (since each mole of hydrogen is 1g)

The oxygen makes up 16g.