Percent Composition

The percent composition is simply a way of relating the relative amounts of elements in a compound.

For instance, a molecule of water (with a molar mass of approximately 18g/mol) contains 2 atoms of hydrogen (molar mass of approximately 1 g/mol). So the percent of hydrogen is:

The percent oxygen could be found in a similar manner

Of course you could also have gotten the percent oxygen by subtraction

 100% - 11.1% = 88.9%

Let's look at a more complicated example.