To find what percentage of a compound is a particular element, you must first calculate the relative formula mass (Mr) of the compound. Then, use the following equation:
Calculate the percentage by mass of hydrogen in ethanol C2H5OH.
Mr of C2H5OH = (2 × 12) + (5 × 1) + (16 + 1) = 46
There are six atoms of hydrogen in the compound, so the mass of hydrogen is 6 × 1 = 6.
Atoms and molecules are too small to count individually. Instead, chemists use a quantity called amount of substance, measured in a unit called the mole (mol).
A mole contains 6 × 1023 particles, a number known as the Avogadro Constant.
For example, one mole of carbon is 12 g and contains 6 × 1023 atoms of carbon. The relative formula mass or water is 18. One mole of water weighs 18 grams and contains 6 × 1023 molecules of water.