An empirical formula shows the simplest whole-number ratio of the atoms of each element in a compound. A formula worked out from reacting masses or from percentage composition is always an empirical formula.
- Write down the mass of each element. If you are given percentages, treat them as masses in 100 g.
- Divide each mass by the relative atomic mass of that element.
- Divide every answer by the smallest number to get the ratio.
- If the ratio is not whole numbers, multiply every number by the same amount until it is.
Example: 2.4 g of magnesium combines with 1.6 g of oxygen. Mg: 2.4 ÷ 24 = 0.1. O: 1.6 ÷ 16 = 0.1. The ratio is 1 : 1, so the formula is MgO.
Example: a hydrocarbon is 80% carbon and 20% hydrogen. C: 80 ÷ 12 = 6.67. H: 20 ÷ 1 = 20. Dividing both by 6.67 gives C = 1 and H = 3, so the empirical formula is CH3.
Example: 5.6 g of iron combines with 2.4 g of oxygen. Fe: 5.6 ÷ 56 = 0.1. O: 2.4 ÷ 16 = 0.15. Dividing by 0.1 gives Fe = 1 and O = 1.5. This is not a whole-number ratio, so multiply both by 2 to get 2 iron to 3 oxygen.