Chemistry 11 · Chemistry · Lesson 4.5
Empirical and molecular formulae
- Level
- Grade 11
- Reading time
- 12 min reading
- Review
- Last reviewed 2026-09-03
- Standard
- Accuracy standard
- Errata
Abstract
Finding the simplest ratio and then the true molecular formula.
What is the difference between empirical and molecular formula?
Short answer
An empirical formula gives the simplest whole-number ratio of atoms in a compound; a molecular formula gives the actual number of each atom in one molecule. The molecular formula is always a whole-number multiple of the empirical formula — for example, hydrogen peroxide is empirical HO but molecular H₂O₂ (twice the empirical unit).
Concept preview
What you will be able to do
- 01Distinguish empirical and molecular formulas
- 02Find an empirical formula from percentages or masses
- 03Find a molecular formula using the molar mass
The empirical formula is the smallest whole-number ratio of the atoms in a compound — its 'simplest formula'. The molecular formula gives the actual number of each atom in a molecule. For example , , and all have the empirical formula (twice as many H as C), but different molecular formulas. Finding an empirical formula is essentially the reverse of finding percentage composition.
School Plan
Empirical and molecular formulae
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Related topics
Frequently asked questions
- How do you find the molecular formula from the empirical formula?
- Divide the compound’s molar mass by the empirical formula mass to get a whole number, then multiply the empirical formula by it.