Empirical Formula

The empirical formula is the simplest whole number ratio of atoms of each element in a compound.

Some substances do not exist as individual molecules. For example sodium chloride is an ionic salt that exists as a giant lattice. The formula NaCl gives the whole number ratio of atoms of sodium and chlorine.NaCl is also called the formula unit when calculating relative masses.
The empirical formula can be calculated using chemical analysis to determine the mass of each element in a compound or the percentage composition by mass of each element in a compound. This can be used to find the number or relative number of moles of each element by dividing the mass or percentage mass by the relative atomic mass. This can then be converted into a whole number ratio in its simplest terms.Example 1:75% by mass of a compound is carbon and 25% by mass is hydrogen.

element	C	H
ratio of moles	75 12	25  1
	6.25	25
simplest whole number ratio	1	4

The empirical formula is CH₄Example 2:2.406g of calcium combines with 4.16g of chlorine to form a compound.

element	Ca	Cl
ratio of moles	2.406  40.1	4.16 35.5
	0.06	0.117
simplest whole number ratio	1	2

The empirical formula of the compound formed is CaCl₂.