Empirical formulae
Extended tier only
- The empirical formula is the simplest whole number ratio of the atoms of each element present in one molecule or formula unit of the compound
- The empirical formula of an organic molecule is often different to its molecular / chemical formula
- For example, ethanoic acid has the chemical formula CH3COOH or C2H4O2 but its empirical formula is CH2O
- The molecular / chemical formula of an ionic compound is always its empirical formula
- For example, sodium chloride has the chemical formula NaCl, which is also its empirical formula
Worked example
Complete the table to give the molecular and empirical formulae of the given compounds.
Chemical | Molecular formula | Empirical formula |
Answers:
The completed table is:
Chemical | Molecular formula | Empirical formula |
C3H8 | C3H8 | |
C2H4 | CH2 | |
C4H10O | C4H10O |
- The first compound contains 3 carbon atoms and 8 hydrogen atoms
- This 3:8 ratio of atoms cannot be simplified
- Therefore, the molecular and empirical formula are both C3H8
- The second compound contains 2 carbon atoms and 4 hydrogen atoms
- This 2:4 ratio of atoms can be simplified to 1:2
- Therefore, the molecular formula is C2H4 and the empirical formula is CH2
- The third compound contains 4 carbon atoms, 10 hydrogen atoms and 1 oxygen atom
- This 10:4:1 ratio of atoms cannot be simplified
- Therefore, the molecular and empirical formula are both C4H10O