Empirical Formula Calculator
The empirical formula of a compound shows the simplest whole-number ratio of its constituent atoms, and it is determined by converting the mass of each element to moles and then dividing by the smallest mole count. This calculator handles C-H-O compounds — the most common in organic chemistry — and returns the mole ratios that define the empirical formula.
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Formula
Ratio = (mass / atomic mass) / min(all mole values)
First, convert each elemental mass to moles by dividing by the atomic mass (C = 12.011, H = 1.008, O = 15.999 g/mol). Then divide each mole count by the smallest mole count to get the simplest ratio. If the ratios are close to whole numbers, round them. If a ratio ends in .5, multiply all ratios by 2; if it ends in .33 or .67, multiply by 3. The resulting whole numbers are the subscripts in the empirical formula.
How to use the Empirical Formula Calculator
- 1
Enter your mass of carbon
Value should be in g.
- 2
Enter your mass of hydrogen
Value should be in g.
- 3
Enter your mass of oxygen
Value should be in g.
- 4
Read your results instantly
Results update in real time as you type.
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Empirical formula vs molecular formula
The empirical formula is the simplest whole-number ratio of atoms in a compound. The molecular formula shows the actual number of each atom in one molecule and is always a whole-number multiple of the empirical formula. For example, glucose (C₆H₁₂O₆) and fructose share the same empirical formula CH₂O — a 1:2:1 ratio of C:H:O — but are entirely different compounds with different structures and properties. To find the molecular formula from the empirical formula, you need the molar mass of the compound (from experiment), which you divide by the empirical formula mass to get the multiplier. Without that experimental molar mass, the empirical formula is the furthest you can go from combustion analysis data alone.
Combustion analysis: the experimental method
Empirical formulas of organic compounds are typically determined by combustion analysis. A known mass of the compound is burned completely in excess oxygen, and the masses of CO₂ and H₂O produced are measured. The mass of carbon is extracted from the CO₂ (each mole of CO₂ contains one mole of C) and the mass of hydrogen from the H₂O (each mole of H₂O contains two moles of H). If the compound also contains oxygen, its mass is found by subtracting the masses of C and H from the original sample mass. These three masses are then divided by atomic masses to give moles, and the mole ratio gives the empirical formula. This method has been used since the early 19th century and remains the foundation of organic elemental analysis.
Tips & Insights
Percentages work too
If you have percent composition instead of masses, use the percentages directly as gram values (assume a 100 g sample). The ratios come out identical because you are still dividing by atomic masses and then normalizing.
Round carefully — 1.5 means multiply by 2
Ratios of 1.5, 2.5, or 3.5 indicate the empirical formula should be scaled by a factor of 2. Ratios near 1.33 or 1.67 suggest multiplying by 3. Only round if the value is within 0.05 of a simple fraction.
Check your masses sum to the sample mass
For a combustion analysis, the masses of C + H + O should equal the total sample mass. If they do not add up, nitrogen, sulfur, or halogens may be present and are not covered by this calculator.
Worked Examples
Glucose from percent composition
C:H:O ratios ≈ 1:2:1 — consistent with the empirical formula CH₂O, which is the simplest ratio for glucose.
Acetic acid
C:H:O ratios ≈ 1:2:1 — same empirical formula CH₂O, illustrating that multiple molecular formulas share one empirical formula.
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Frequently Asked Questions
What is an empirical formula?
An empirical formula shows the simplest whole-number ratio of atoms in a compound. It does not reveal the actual number of atoms per molecule — that requires the molecular formula.
How is the empirical formula different from the molecular formula?
The molecular formula gives the actual count of each atom in a molecule and is a whole-number multiple of the empirical formula. For example, benzene's empirical formula is CH, but its molecular formula is C₆H₆.
What do I do if a ratio comes out as 1.5?
Multiply all ratios by 2. Empirical formulas must have whole-number subscripts, so a ratio of 1.5 becomes 3 after multiplying by 2, and a ratio of 1 becomes 2.
Can this calculator handle nitrogen-containing compounds?
No. This calculator covers only C, H, and O. For compounds containing nitrogen, phosphorus, sulfur, or halogens, add those elements' contributions manually after computing the C-H-O ratios.
How do I find the molecular formula from the empirical formula?
Determine the molar mass of the compound experimentally (e.g., by mass spectrometry). Divide it by the empirical formula mass to get the integer multiplier n, then multiply each subscript in the empirical formula by n.
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