Empirical Formula
The simplest whole-number ratio of atoms in a compound.
About this topic
The empirical formula is the simplest whole-number ratio of atoms in a compound. To find it from percent composition or mass data: convert masses to moles, divide all by the smallest number of moles, and, if needed, multiply to reach whole numbers (1.5 becomes 3:2 after doubling).
Experiments such as burning a hydrocarbon and weighing the carbon dioxide and water produced give the masses of each element. Percentages can be treated as grams out of 100 g. After dividing by the smallest mole value, ratios near 1.25 suggest multiplying by 4, near 1.33 by 3, near 1.5 by 2, and near 1.67 by 3. Small rounding errors are fine, but do not round 1.5 to 2.
Worked example: a compound is 40.0 % carbon, 6.7 % hydrogen and 53.3 % oxygen. In 100 g that is 3.33 mol C, 6.65 mol H and 3.33 mol O. Dividing by 3.33 gives 1 : 2 : 1, so the empirical formula is CH2O. Glucose, acetic acid and formaldehyde all share it but have different molecular formulas.
Empirical formulas are the only formulas used for ionic compounds and giant structures. A common mistake is using percentages directly as ratios without converting to moles, or forgetting to divide by the smallest value.
Key ideas
- Convert masses to moles
- Divide by the smallest
- Round to whole numbers
Quick check
Use cases: where Empirical Formula helps
- Homework: review the key idea behind Empirical Formula before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Empirical Formula without notes.
- Visual learning: play with the interactive lab to see how Empirical Formula changes when you adjust its inputs.
- Teaching a friend: explain Empirical Formula in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Empirical Formula?
The simplest whole-number ratio of atoms in a compound.
What will I learn in Empirical Formula?
Convert masses to moles. Divide by the smallest. Round to whole numbers.
Is Empirical Formula part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Empirical Formula lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Empirical Formula?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.