Empirical and Molecular Formulae

The empirical formula of a compound tells you the simplest whole-number ratio of atoms of each element present. The molecular formula tells you the actual number of atoms of each element in one molecule. These two formulae are sometimes the same (for example, water is both H₂O), but often the molecular formula is a whole-number multiple of the empirical formula.

---

Empirical Formula from Percentage Composition

To find the empirical formula from percentage composition data, follow these steps:

1. Write down the percentage (or mass) of each element.
2. Divide each by the element's relative atomic mass (Aᵣ) to find the number of moles.
3. Divide each mole value by the smallest mole value to get the simplest ratio.
4. If the ratio is not whole numbers, multiply all values by the same factor to get whole numbers.

Worked Example 1

A compound contains 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. Find its empirical formula.