Empirical and Molecular Formulae
Use this landing page to move quickly between the key empirical and molecular formulae revision pages for Topic 5. Each card links to a focused page covering the simplest whole-number ratio of atoms and how to use molar mass to find the actual molecular formula.
Choose an Empirical and Molecular Formulae Page
Empirical and molecular formulae connect experimental composition data with the actual formula of a compound. These pages build from finding the simplest whole-number ratio of atoms through to using relative molecular mass to scale up to the true molecular formula.
Key idea: The empirical formula gives the simplest whole-number ratio of atoms in a compound, while the molecular formula gives the actual number of each type of atom in one molecule.
Formulae
Empirical Formulae
Revise how to convert masses, percentages or combustion data into mole ratios and then into the simplest whole-number formula.
Formulae
Molecular Formulae
Learn how the empirical formula mass and relative molecular mass are used to find the molecular formula of a compound.
How to Use These Pages
Start with empirical formulae so that you can confidently convert composition data into a ratio of moles. Then move to molecular formulae, where the empirical formula is treated as a repeat unit and scaled using the relative molecular mass.
Exam focus: In empirical formula calculations, divide each amount in moles by the smallest value before converting to whole numbers. In molecular formula calculations, compare the relative molecular mass with the empirical formula mass.
Master Formulae, Equations and Amounts of Substance for Edexcel A Level Chemistry
Continue from these free revision notes into the full Topic 5 Formulae, Equations and Amounts of Substance course, with guided video teaching, diagnostic MCQ practice, teacher-marked short-answer questions and a specification assignment with a personalised progress report.
Guided video teaching
Learn formulae, balanced equations, ionic equations, relative masses, mole calculations, empirical formulae, reacting masses, gas volumes, percentage yield and atom economy through structured video lessons with worked examples and walkthroughs.
Instant MCQ feedback
Auto-marked MCQ quizzes provide immediate diagnostic feedback for every answer choice.
Teacher-marked SAQs
Submit written exam responses and receive chemistry specialist feedback with improvement guidance.
Progress tracking
Identify strengths and weaknesses across formulae, equations, mole calculations, concentration, empirical formulae, percentage yield, atom economy and test-tube reactions with targeted reporting.
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A compound contains carbon, hydrogen and oxygen only. Its percentage composition by mass is shown.
| Element | Percentage by mass |
|---|---|
| C | 40.0% |
| H | 6.7% |
| O | 53.3% |
What is the empirical formula of the compound?
Explain how the volume of hydrogen can be used to calculate the amount of magnesium that reacted.
Copyright notice: This revision page, including its written explanations, layout and teaching sequence, is produced for Online Learning System by Dr. Mohammed Al-Fatah. It is intended for student revision and may not be copied, redistributed or republished without permission.