The empirical (experimental) formula is the simplest whole number ratio of atoms in a compound.
The word empirical means experimental, and empirical formulae are deduced from experimentally-determined masses of elements present in a particular sample of the compound. The data is usually expressed as a mass percentage.
| Any mass of H2O has | 11.1 mass % H | 89.1 mass % O |  | ||
| 100 g H2O contains | 11.1 g H | and 89.1 g O | mass in grams to | divide the mass in grams (m) by the molar mass of the element (M) THEN | |
| 100 g H2O contains | 11.1 mol H | and 5.55 mol O | amount in moles to | ||
| Any sample of water has this ratio of amounts in moles of H to O | 2 mol H | and 1 mol O | mole ratio to | divide each amount in moles by the smallest amount in moles to get the subscripts | |
| H2O | formula | ||||
- Note that subscripts in empirical formulae are integers. If the calculated ratio is not an integer, both parts of the ratio should be multiplied by the same factor in order to get the smallest whole number ratio.
Example where this is required:
Using the mass % of N and O in N2O5 gives the ratio of the amount in moles n(N) to n(O) of 1 mol N to 2.5 mol O.
Both should be multiplied by 2 to give n(N) to n(O)of 2 mol N to 5 mol O