# Empirical and molecular formula

The empirical formula is the simplest whole-number ratio of atoms in a compound. The molecular formula is the actual number of atoms. Molecular formula = empirical formula × n, where n = molar mass ÷ empirical mass.

Chemistry (university) · Part 04: Stoichiometry and reactions in solution · Card 2 of 9 · Last updated 2026-10-19

## Analogy

A recipe scaled down to its smallest form: 'two parts flour to one part sugar' hides how big the batch is.

## Example

**40.0% C, 6.7% H, 53.3% O**

Per 100 g: C 3.33 mol, H 6.6 mol, O 3.33 mol
Divide by 3.33: C 1, H 2, O 1
Empirical formula: CH₂O

## In practice

Combustion analysis turns burnt mass of CO₂ and H₂O into an empirical formula for an unknown organic compound.

## Practice

### Flashcards

- **What is the empirical formula?** The simplest whole-number ratio of atoms in a compound.

## Questions people ask

### How do I find the molecular formula from the empirical formula?

Divide the compound's molar mass by the empirical formula mass to get a whole number n. Multiply every subscript by n. If CH₂O (30 g/mol) has a measured molar mass of 180 g/mol, n = 6, so the molecular formula is C₆H₁₂O₆, which is glucose.

### What if my ratios are not whole numbers?

Multiply all the ratios by the same small number to make them whole. A ratio of 1 : 1.5 becomes 2 : 3 when doubled. A ratio of 1 : 1.33 becomes 3 : 4 when tripled. Only round if the value is within about 0.1 of a whole number.

## Related concepts

- [Mole and molar mass](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-4/chemistry-university-part-4-concept-1)

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