Empirical & Molecular Formulae
Chemistry: AQA A Level, topic 1.2.4
Empirical and Molecular Formulae
The empirical formula is the simplest formula for a compound. The molecular formula gives the number and types of atoms in a compound.
Empirical formula
- The empirical formula is the simplest whole number ratio of the atoms in a molecule.
- E.g. Ethane has two carbon atoms and six hydrogen atoms.
- CH3 is the empirical formula for ethane.
Molecular formula
- A molecular formula is the actual number of atoms of each element in a molecule. It is always a multiple of the empirical formula.
- E.g. Ethane has two carbon atoms and six hydrogen atoms.
- C2H6 is the molecular formula of ethane.
- This is 2x its empirical formula.
Experimental determination
- The empirical formula of a compound is often very easy to determine experimentally.
- For instance, you could combust a hydrocarbon and find that you get twice as much water vapour as carbon dioxide.
- This would tell you that its empirical formula is CH4 (remember H2O contains two H atoms).
- A molecular formula is relatively difficult to determine experimentally.
Test yourself on Empirical & Molecular Formulae
What is the empirical formula for ethane?
What is the molecular formula for ethane?