Empirical and molecular formulae · Grade 11
An empirical formula is the simplest whole-number ratio of atoms in a compound. You find it from the masses or percentages of the elements: work out the moles of each element, then divide by the smallest to get the ratio. If you also know the Mᵣ, you can scale the empirical formula up to the molecular formula: the actual number of each atom in one molecule.
Key facts
- Empirical formula: the simplest whole-number ratio of atoms (ethane, C2H6, has the empirical formula CH3).
- Molecular formula: the actual number of each atom in one molecule.
- Method: mass of each element → moles (÷ Aᵣ) → divide by the smallest → whole numbers.
- With percentages, imagine 100 g of the compound: the percentages become masses in grams.
- If a ratio ends in .5, double every number; if it ends in .33 or .67, multiply every number by 3.
- Molecular formula = empirical formula × (Mᵣ ÷ the mass of one empirical formula unit).