Finding the percentage of an element in a compound by mass

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:

{\%~of~an~element} = \frac{mass~of~an~element~in~the~compound}{relative~formula~mass~M_r} \times 100


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.

{\%~of~hydrogen} = \frac{mass~of~hydrogen}{relative~formula~mass} \times 100 = \frac{6}{46} \times 100 = 13\%

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.

The mass of one mole of any substance is numerically equal to its relative formula mass or relative atomic mass

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.