Empirical Formula Calculator
Find the simplest whole-number atom ratio from percentages or masses.
Smallest whole-number ratio of atoms.
| Element | Mass % | Moles | ÷ smallest | Whole |
|---|---|---|---|---|
| C | 40 | 3.3303 | 1 | 1 |
| H | 6.7 | 6.6468 | 1.996 | 2 |
| O | 53.3 | 3.3315 | 1 | 1 |
To find an empirical formula, convert each element’s mass or percentage to moles (mass ÷ atomic mass), divide every value by the smallest, then scale to whole numbers. A compound of 40.0% C, 6.7% H, 53.3% O gives 3.33 : 6.65 : 3.33 → 1 : 2 : 1 → CH₂O.
From composition to formula
The empirical formula is the simplest whole-number ratio of atoms in a compound. The method is always the same: convert each element’s percentage or mass to moles, divide every value by the smallest, and scale to whole numbers.
then divide every mole value by the smallest to get the ratio
Worked example
A compound is 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. Find its empirical formula.
- 1 Treat percentages as grams. Assume a 100 g sample: 40.0 g C, 6.7 g H, 53.3 g O.
- 2 Convert each to moles. C: 40.0 ÷ 12.011 = 3.33. H: 6.7 ÷ 1.008 = 6.65. O: 53.3 ÷ 15.999 = 3.33.
- 3 Divide by the smallest. 3.33 ÷ 3.33 = 1 (C), 6.65 ÷ 3.33 ≈ 2 (H), 3.33 ÷ 3.33 = 1 (O) → CH₂O.
Empirical vs molecular formulas
The molecular formula is a whole-number multiple of the empirical formula.
| Compound | Empirical | Molecular |
|---|---|---|
| Glucose | CH₂O | C₆H₁₂O₆ |
| Benzene | CH | C₆H₆ |
| Hydrogen peroxide | HO | H₂O₂ |
| Acetic acid | CH₂O | C₂H₄O₂ |
| Water | H₂O | H₂O |
From empirical to molecular
You need the molar mass too. The empirical formula gives only the ratio. To get the molecular formula, divide the known molecular mass by the empirical formula mass, then multiply every subscript by that whole number.
For CH₂O, the empirical mass is 12.011 + 2×1.008 + 15.999 = 30.026 g/mol. Glucose has a molar mass of 180.16 g/mol, so 180.16 ÷ 30.026 ≈ 6, giving C₆H₁₂O₆.
Common mistakes. Rounding moles too aggressively, ignoring ratios near 2.5 or 1.33 that need a ×2 or ×3 scale-up, and forgetting that a ratio close to a whole number (like 1.96) really is that integer.