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Empirical Formula Calculator
Enter the percent composition (or grams) of each element. The calculator converts to moles, divides by the smallest value, finds the whole-number ratio and, if you give a molar mass, the molecular formula. Every step is shown.
Enter mass percentages or grams from an analysis. They do not need to add up to 100.
How to use the empirical formula calculator
- Enter each element and its mass percent (or the grams found by analysis). Assume a 100 g sample, so 40.0% C means 40.0 g C.
- The calculator divides each mass by the element's atomic mass to get moles.
- It divides every mole value by the smallest one, then multiplies by 2, 3… if needed to reach whole numbers (e.g. 1.5 → ×2).
- Optionally enter the compound's molar mass. n = molar mass ÷ empirical formula mass gives the molecular formula.
Worked examples
40.0% C, 6.70% H, 53.3% O
C 3.330 mol, H 6.647 mol, O 3.332 mol
÷ 3.330 → 1 : 1.996 : 1.000
Empirical CH2O (30.026 g/mol)
M = 180 → n = 6 → C6H12O6
64.8% C, 13.6% H, 21.6% O
C 5.395, H 13.49, O 1.350 mol
÷ 1.350 → 3.996 : 9.993 : 1
C4H10O
(e.g. butanol or diethyl ether, 74.12 g/mol)
69.94% Fe, 30.06% O
Fe 1.2524, O 1.8789 mol → 1 : 1.500
× 2 → 2 : 3
Fe2O3
Empirical vs 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 in one molecule, always a whole-number multiple of it. Glucose (C6H12O6), formaldehyde (CH2O) and acetic acid (C2H4O2) all share the empirical formula CH2O.
moles = mass ÷ atomic mass → ratio = moles ÷ smallest moles → n = M(compound) ÷ M(empirical)
Handling ratios that are not whole numbers
| Ratio ends in about… | Multiply all by |
|---|---|
| .5 | 2 |
| .33 or .67 | 3 |
| .25 or .75 | 4 |
| .2, .4, .6, .8 | 5 |
Small deviations (±0.1) come from rounding in experimental data and are rounded away. Large deviations usually mean a wrong percentage or a missing element: if the percentages add to less than 100, the remainder is often oxygen.
Going the other direction, formula to percent composition, is done by the molecular weight calculator.
Found the molecular formula? Draw the possible isomers in MolDraw to check which structure fits your spectra.
Draw isomers in MolDrawFrequently asked questions
How do you calculate an empirical formula from percent composition?
Treat the percentages as grams in a 100 g sample, convert each to moles, divide by the smallest number of moles and round (or multiply up) to whole numbers.
How can I calculate the molecular formula from the empirical formula?
Divide the compound's molar mass by the empirical formula mass to get n, then multiply every subscript by n. CH2O (30.03) with M = 180 gives n = 6, C6H12O6.
What is the empirical formula of a compound with 40.0% C, 6.70% H and 53.3% O?
CH2O. The mole ratio is 3.33 : 6.65 : 3.33, which simplifies to 1 : 2 : 1.
What is the empirical formula of a compound that is 64.8% C, 13.6% H and 21.6% O?
C4H10O. Moles: C 5.40, H 13.5, O 1.35. Dividing by 1.35 gives 4 : 10 : 1.
What if the percentages do not add up to 100?
The calculator works with any amounts, because only the ratio matters. If an element is missing from the analysis (often oxygen), add it as 100 minus the sum.
Can the empirical and molecular formula be the same?
Yes, when n = 1. Water (H2O) and carbon dioxide (CO2) are examples.
Related chemistry tools
Need to sketch molecules for a report? Draw chemical structures online with MolDraw, a free online ChemDraw alternative (see the ChemDraw vs MolDraw comparison).