Combustion Analysis Calculator (empirical formula from CO₂ and H₂O masses)

Understanding the combustion analysis calculator — concept and principle

Combustion analysis finds the empirical formula of an organic compound by burning a weighed sample and collecting the carbon dioxide and water produced. This calculator converts those masses into element percentages, the empirical formula and, with a molar mass, the molecular formula.

All carbon ends up in CO₂ and all hydrogen in water; the rest is oxygen, giving the formula C₂H₆O.

How it is calculated

Carbon: mC = mCO₂ × 12.011 ÷ 44.009. Hydrogen: mH = mH₂O × 2 × 1.008 ÷ 18.015.
Oxygen by difference: sample mass − C − H (taken as zero if under 0.5% of the sample).
Moles of each = mass ÷ atomic weight; divide by the smallest, then multiply by 1, 2, 3 … 12 until all are within 0.1 of whole numbers.
Molecular formula: n = molar mass ÷ empirical formula mass (rounded).

The principle

Burning in excess oxygen converts every carbon atom to CO₂ and every hydrogen atom to H₂O, which are trapped and weighed separately. Because each CO₂ contains one carbon atom, the carbon mass is the CO₂ mass times the fraction of carbon in CO₂ (12.011 ÷ 44.009); likewise for hydrogen in water. Oxygen in the sample cannot be measured this way because oxygen gas is also consumed, so it is found by difference.

Worked example

Default inputs: sample 10.00 mg, CO₂ 19.11 mg, H₂O 11.73 mg, molar mass 46.07 g/mol.

C = 19.11 × 12.011 ÷ 44.009 ≈ 5.216 mg (52.16%); H = 11.73 × 2.016 ÷ 18.015 ≈ 1.313 mg (13.13%).
O = 10.00 − 5.216 − 1.313 ≈ 3.472 mg (34.72%).
Moles: C 0.4342, H 1.302, O 0.2170 mmol → ratio 2.00 : 6.00 : 1.00.
Empirical formula = C₂H₆O, mass 46.069 g/mol.
n = 46.07 ÷ 46.069 = 1, so the molecular formula is also C₂H₆O (ethanol or dimethyl ether).

How to read the result

The formula tells you composition, not structure: C₂H₆O could be ethanol or dimethyl ether. Spectroscopy (IR, NMR, MS) is needed to identify the compound. If ratios do not come close to whole numbers, check the masses for weighing or trapping errors.

Common mistakes

Using the mass of H₂O as hydrogen mass without the 2.016 ÷ 18.015 factor.
Rounding mole ratios too early (1.5 must be multiplied by 2, not rounded to 2).
Forgetting that oxygen is found by difference.
Applying the method to compounds containing N, S or halogens without extra analyses.

Keep in mind: Small weighing errors strongly affect the formula. Real elemental analysis uses calibrated instruments and duplicate runs.

Frequently asked questions

How do you find an empirical formula from combustion analysis?

Convert CO₂ to carbon and H₂O to hydrogen, find oxygen by difference, convert to moles and find the smallest whole-number ratio.

Why is oxygen found by difference?

Oxygen is supplied for the combustion, so oxygen in the products does not all come from the sample.

What is the difference between empirical and molecular formulas?

The empirical formula is the simplest ratio; the molecular formula is a whole-number multiple of it.

Sources: Zumdahl, S. S. & Zumdahl, S. A., Chemistry, 10th ed. Cengage (combustion analysis) · IUPAC, Standard atomic weights (C 12.011, H 1.008, O 15.999)

Checked 9 Nov 2026. Combustion analysis: mass of C from CO₂ and of H from H₂O, oxygen by difference, mole ratio → empirical formula, molar mass → molecular formula (standard stoichiometry); atomic weights C 12.011, H 1.008, O 15.999 (standard abridged values).

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