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To find what percentage of a compound is a particular element, you must first calculate the relative formula mass (M_r) of the compound. Then, use the following equation:

Example:

Calculate the percentage by mass of hydrogen in ethanol C₂H₅OH.

Answer:

M_r 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.