What is called the mass fraction of an element in a complex substance? how is this value calculated?

A molecule of a complex substance consists of a certain number of different atoms. Consequently, the mass of a molecule is equal to the sum of the masses of all the atoms that make up its composition. The fraction of atoms of each element in a molecule of a complex substance is calculated by the formula:

mass fraction of an element in a substance molecule = (relative atomic mass of an element * number of atoms of an element in a molecule): mass of a substance molecule * 100%.

For example, let’s find the mass fraction of oxygen in water (H2O):

1) The mass of a water molecule = atomic mass of oxygen + atomic mass of hydrogen * 2 = 16 + 1 * 2 = 18.

2) Mass fraction of oxygen in H2O = 16: 18 * 100% = 88.88%.