Examples:
Empirical formula: CH2O
Molecular formula: C6H12O6
| Element | Amount | ÷ Atomic mass | = Moles | ÷ Smallest | Subscript |
|---|---|---|---|---|---|
| C | 40 g* | 12.011 | 3.3303 | 1.000 | 1 |
| H | 6.71 g* | 1.008 | 6.6567 | 1.999 | 2 |
| O | 53.29 g* | 15.999 | 3.3308 | 1.000 | 1 |
- *Assume a 100 g sample, so each percentage becomes that many grams.
- Divide each mass by the element's atomic mass to get moles.
- Divide every mole value by the smallest one to get the ratio.
- Empirical formula mass:
30.026 g/mol - Molar mass ÷ empirical mass = 180.16 ÷ 30.026 ≈
6, so multiply every subscript by 6.
How it works
Each amount is divided by the element's standard atomic weight to get moles, and every mole value is divided by the smallest. If the resulting ratios aren't all within 0.1 of a whole number, the calculator tries multiplying them by 2, 3, 4… (up to 12) until they are — which is how ratios like 1 : 1.5 become Fe₂O₃. Formulas are written in Hill order: carbon first, hydrogen second, then everything else alphabetically.
Real lab data carries measurement error, so if no small multiplier gives clean whole numbers, the tool says so rather than forcing a guess.
Frequently asked questions
- What is an empirical formula?
- The simplest whole-number ratio of atoms in a compound. Glucose, C₆H₁₂O₆, has the empirical formula CH₂O, because 6 : 12 : 6 reduces to 1 : 2 : 1.
- How do you find an empirical formula from percentages?
- Assume a 100 g sample so each percentage becomes grams, convert each mass to moles by dividing by the atomic mass, divide every mole value by the smallest one, and if the ratios are not whole numbers, multiply them all by the smallest factor that makes them whole.
- What if a ratio comes out as 1.5 or 1.33?
- Those are fractions in disguise: 1.5 is 3/2, so multiply every ratio by 2; 1.33 is 4/3, so multiply by 3. Do not round 1.5 to 2 — that gives the wrong formula.
- How do you get the molecular formula?
- Divide the compound’s molar mass by the empirical formula mass. The result should be close to a whole number n; multiply every subscript in the empirical formula by n.
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