Empirical & Molecular Formulae (Edexcel IGCSE Chemistry)

• The molecular formula is the formula that shows the number and type of each atom in a molecule
  • E.g. the molecular formula of ethanoic acid is C₂H₄O₂

• 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
  • E.g. the empirical formula of ethanoic acid is CH₂O

• Organic molecules often have different empirical and molecular formulae
• The formula of an ionic compound is always an empirical formula

Empirical formula: gives the simplest whole number ratio of atoms of each element in the compound

• It is calculated from knowledge of the ratio of masses of each element in the compound

Example: A compound that contains 10 g of hydrogen and 80 g of oxygen has an empirical formula of H₂O. This can be shown by the following calculations:

Amount of hydrogen atoms = mass in grams ÷ A_r of hydrogen = (10 ÷ 1) = 10 moles
Amount of oxygen atoms = mass in grams ÷ A_r of oxygen = (80 ÷ 16) = 5 moles

The Ratio of Moles of Hydrogen Atoms to Moles of Oxygen Atoms

Since equal numbers of moles of atoms contain the same number of atoms, the ratio of hydrogen atoms to oxygen atoms is 2:1. Hence the empirical formula is H₂O
Molecular formula: gives the exact numbers of atoms of each element present in the formula of the compound

• Divide the relative formula mass of the molecular formula by the relative formula mass of the empirical formula
• Multiply the number of each element present in the empirical formula by this number to find the molecular formula

Relationship between Empirical and Molecular Formula