Combustion Analysis Calculator
Oxygen is found by difference, so every error lands on it.
Work out Combustion Analysis. Oxygen is found by difference, so every error lands on it. Free, with no account and nothing to install.
Empirical formula
C5H5O3
From 0.9776 g CO₂ and 0.2001 g H₂O
Oxygen is obtained by difference rather than measured, so every error in the sample and product masses collects in it. This gives the empirical formula only — the molecular formula needs the molar mass as well.
How the Combustion Analysis Calculator works
Works an empirical formula back from the carbon dioxide and water a sample produces on burning, showing the mass of each element before the ratio. Oxygen never gets measured — it is whatever mass is unaccounted for — which is worth seeing rather than being told.
Also known as: empirical formula from co2 and water · elemental analysis formula finder · burning a sample to find its formula · chn analysis calculator
Why the products carry the answer
Burn an organic compound in excess oxygen and every carbon atom leaves as CO₂ and every hydrogen as H₂O. Trap and weigh those two products and you have measured the carbon and hydrogen in the original sample without ever isolating either element.
The conversions are pure mass fractions. Carbon is 12.011 of CO₂'s 44.009 g/mol, so multiply the CO₂ mass by 0.2729. Hydrogen is 2.016 of H₂O's 18.015, so multiply the water mass by 0.1119 — note that both hydrogens count, which is why the numerator is 2.016 rather than 1.008.
The classic apparatus does the trapping in sequence: a magnesium perchlorate tube absorbs water first, then a soda-lime tube absorbs carbon dioxide, and each is weighed before and after. Order matters, because soda lime absorbs water as well and would take both if it came first.
Oxygen by difference, and what that costs
Oxygen cannot be measured directly by this method, because the combustion runs in excess oxygen and the products contain oxygen from both the sample and the supply. There is no way to tell them apart. So oxygen is whatever the sample mass has left after carbon and hydrogen are subtracted.
That makes oxygen the error sink. Every uncertainty in the sample mass and in both product masses accumulates in it, with none of its own measurement to average against. If the carbon and hydrogen figures each carry 1% error, the oxygen figure can carry considerably more, and the smaller the oxygen fraction the worse the relative error becomes.
A negative oxygen mass is the failure this produces most visibly, and it is unmistakable: carbon and hydrogen together cannot outweigh the sample they came from. It usually means the sample was weighed light or a product tube picked up moisture from the air before its final weighing.
What the method cannot see
Nitrogen, sulfur, halogens and metals all break this calculation quietly. Their mass is in the sample and is not in the CO₂ or the H₂O, so the arithmetic assigns it to oxygen. A compound containing nitrogen analysed as though it were CHO comes back with an oxygen count that is simply wrong, with nothing in the numbers to flag it.
This is what a CHN analyser is for: it measures nitrogen separately by reducing the combustion products and quantifying N₂, so nitrogen leaves the oxygen-by-difference figure. Sulfur and halogens need their own determinations again — usually ion chromatography or oxygen flask combustion.
And even done perfectly the method gives only the empirical formula. Benzene and acetylene both analyse as CH; ethene and cyclohexane both as CH₂. Without a molar mass measurement alongside it, combustion analysis cannot tell them apart.
Where to go next
The Combustion Analysis question rarely arrives on its own. These are the ones that usually come with it:
- Empirical Formula Calculator — From percentages to formula, without rounding 1.5 to 2.
- Molecular Formula Calculator — A ratio that is not near a whole number means bad data.
- Molar Mass Calculator — Any formula, with brackets, hydrates and percent composition.
- Speed Distance Time Calculator — Any one of the three from the other two, in four units.
Frequently asked questions
How does combustion analysis find a formula?
All the carbon in the sample ends up in CO₂ and all the hydrogen in H₂O. Weighing those two products gives the mass of carbon and hydrogen, and anything left over from the sample mass is oxygen.
Why is oxygen calculated by difference?
Because the combustion is run in excess oxygen, so oxygen in the products comes from both the sample and the supply and cannot be separated. Subtracting carbon and hydrogen from the sample mass is the only route to it.
What if the oxygen mass comes out negative?
Something is wrong with the measurements. Carbon and hydrogen together cannot outweigh the sample they came from, so either the sample was weighed light or one of the products was weighed heavy.
Does this work for compounds containing nitrogen or sulfur?
Not on its own. A CHN analyser measures nitrogen separately, and without that measurement the nitrogen mass is wrongly absorbed into the oxygen figure.
Does combustion analysis give the molecular formula?
No, only the empirical formula. Benzene and acetylene both analyse as CH; separating them needs the molar mass as well.
Why convert CO₂ mass to carbon mass?
Because only 12.011 of CO₂'s 44.009 g/mol is carbon. Multiplying the CO₂ mass by 12.011/44.009 — about 27.3% — extracts the carbon that came from the sample.
Why weigh the absorption tubes rather than the gases?
Because a mass gain is far easier to measure accurately than a gas volume. The tubes are weighed before and after, and the difference is the mass of product absorbed — no pressure or temperature correction needed.
Does the order of the absorption tubes matter?
Yes. The water absorber must come first, because the carbon dioxide absorber — usually soda lime — takes water as well. Reversing them puts both products in one tube and loses the split entirely.
How much sample does the method need?
A modern instrument works on 1 to 3 mg, weighed on a microbalance. Classical apparatus needed a few hundred milligrams. Either way, the sample must be dry and homogeneous, since surface moisture is counted as hydrogen.
How do I check an analysis against a proposed formula?
Calculate the theoretical percentage of each element from the formula and compare. Agreement within 0.4% absolute is the conventional standard for publication, and a larger deviation usually means residual solvent rather than a wrong structure.
Why does a compound with water of crystallisation analyse badly?
Because the water contributes hydrogen and oxygen that the anhydrous formula does not account for. Analyses of hydrates are quoted against the hydrated formula, and a sample of uncertain hydration state cannot be analysed meaningfully at all.
Can this be used on inorganic compounds?
Only for the carbon and hydrogen they contain. Metals leave a residue rather than a gas, and that residue mass would be assigned to oxygen by this calculation. Inorganic analysis uses different methods for the metal content.
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