Two ways to describe composition:
1- By number of its constituent atoms.
2- Percentage (by mass) of its elements: obtained by comparing the mass of element present in 1 mole of compound to the mass of 1 mole of compound.
The mass percent (often called the weight percent) of carbon in ethanol can be computed by comparing the mass of carbon in 1 mole of ethanol to the total mass of 1 mole of ethanol and multiplying the result by 100:
Note: The total percentage of all elements is 100.00%
Example 2: Carvone is a substance that occurs in two forms having different arrangements of the atoms but the same molecular formula (C10H14O) and mass. One type of carvone gives caraway seeds their characteristic smell, and the other type is responsible for the smell of spearmint oil. Compute the mass percent of each element in carvone.