Empirical Formula Calculator
Enter the percent (or grams) of each element. Add a molar mass to get the molecular formula too.
| Element | Mass (g) | ÷ atomic mass = mol | ÷ smallest | × 1 → subscript |
|---|---|---|---|---|
| C | 40 | 3.33 | 1.000 | 1 |
| H | 6.71 | 6.657 | 1.999 | 2 |
| O | 53.29 | 3.331 | 1.000 | 1 |
Worked example: glucose
Analysis gives 40.0% C, 6.71% H and 53.29% O. In 100 g: C 40.0 ÷ 12.011 = 3.33 mol; H 6.71 ÷ 1.008 = 6.66 mol; O 53.29 ÷ 15.999 = 3.33 mol. Divide by 3.33: 1 : 2 : 1, so the empirical formula is CH₂O (30.03 g/mol). The molar mass is 180.16, and 180.16 ÷ 30.03 = 6, so the molecular formula is C₆H₁₂O₆.
Going the other way, from formula to percentages, is what the molar mass calculator’s mass % column does.
Questions students ask
How do you find an empirical formula?
Treat percentages as grams (assume a 100 g sample). Divide each mass by its atomic mass to get moles. Divide every mole value by the smallest one. If the ratios are not close to whole numbers, multiply them all by a small integer (2, 3, 4…) until they are.
What is the difference between an empirical and a molecular formula?
The empirical formula is the simplest whole-number ratio of atoms (CH₂O). The molecular formula is the actual number of each atom in one molecule (C₆H₁₂O₆ for glucose), always a whole-number multiple of the empirical formula.
How do I get the molecular formula?
Divide the molar mass (from a mass spectrum or other measurement) by the empirical formula mass. Multiply every subscript by that whole number.
What if a ratio comes out as 1.5 or 1.33?
Those are ×2 and ×3 fractions. 1.5 → multiply everything by 2; 1.33 or 1.67 → by 3; 1.25 → by 4. The calculator finds the smallest multiplier automatically.