Combustion Analysis Calculator
Enter the sample mass and the CO2 and H2O collected from a combustion train to get percent C, H, and O, then the empirical formula.
🔥 What is Combustion Analysis?
Combustion analysis is a laboratory technique chemists use to determine the elemental composition of an unknown organic compound. A weighed sample is burned completely in a stream of oxygen inside a combustion train, and the carbon dioxide and water vapor produced are trapped in separate absorbers and weighed. Because every carbon atom in the original sample ends up bound into a CO2 molecule, and every two hydrogen atoms end up bound into an H2O molecule, those two trapped masses can be converted mathematically back into the mass of carbon and hydrogen that was present before burning.
This is one of the oldest and most reliable methods in organic chemistry for identifying an unknown substance. It was the technique Justus von Liebig refined in the 1830s to analyze organic compounds systematically, and modern CHN/CHNS elemental analyzers still use the same underlying principle, just with automated combustion, gas chromatography, and thermal conductivity detection instead of manual weighing. It is routinely used in pharmaceutical quality control to confirm a drug's purity, in petrochemical labs to characterize new fuel blends, and in academic organic chemistry courses to teach the link between raw lab data and molecular structure.
A common point of confusion is thinking this calculator is the same as a generic percent-composition calculator. It is not. Combustion analysis measures carbon and hydrogen indirectly, through the masses of CO2 and H2O, and any oxygen present in the original sample must be found by difference, subtracting the recovered carbon and hydrogen mass from the total sample mass, since a standard combustion train does not trap or weigh oxygen directly.
This calculator performs exactly that specialized conversion: from sample mass, CO2 mass, and H2O mass, it recovers percent carbon, percent hydrogen, and percent oxygen by difference, then reduces the resulting mole ratio to the simplest whole-number empirical formula, with every intermediate step shown.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — An Oxygen-Containing Compound (Glucose)
Burning a 0.5000 g organic sample produces 0.7328 g CO2 and 0.3000 g H2O.
Example 2 — A Pure Hydrocarbon (Octane)
Burning a 0.3000 g fuel sample produces 0.9246 g CO2 and 0.4258 g H2O.
Example 3 — A Compound With a Different C:H:O Ratio (Aspirin)
Burning a 0.4000 g organic sample produces 0.8794 g CO2 and 0.1600 g H2O.
❓ Frequently Asked Questions
🔗 Related Calculators
What is combustion analysis in chemistry?
Combustion analysis is a lab technique for finding the elemental composition of an organic compound. A known mass of the compound is burned completely in oxygen, and the carbon dioxide and water it produces are trapped and weighed separately. Because every carbon atom in the sample becomes part of a CO2 molecule and every two hydrogen atoms become part of an H2O molecule, those two masses can be converted back into the mass of carbon and hydrogen in the original sample.
How do you find percent carbon from the mass of CO2 produced?
Multiply the mass of CO2 by the ratio of carbon's atomic weight to CO2's molar mass (12.011 divided by 44.009), which gives the mass of carbon in the original sample. Divide that by the sample mass and multiply by 100 to get percent carbon. For example, 0.7328 g of CO2 contains 0.7328 times (12.011 divided by 44.009), about 0.2000 g of carbon.
How do you find percent hydrogen from the mass of H2O produced?
Multiply the mass of H2O by the ratio of two hydrogen atomic weights to water's molar mass (2 times 1.008, divided by 18.015), which gives the mass of hydrogen in the original sample. Divide by the sample mass and multiply by 100 for percent hydrogen. Every water molecule carries two hydrogen atoms from the original sample, which is why the factor uses 2 times 1.008, not just 1.008.
Why is oxygen found by difference instead of measured directly?
A standard combustion train only traps and weighs CO2 and H2O, so it directly measures carbon and hydrogen but not oxygen. If oxygen is present in the sample, its mass is recovered indirectly by subtracting the measured carbon and hydrogen mass from the total sample mass. Any mass left over after that subtraction is assumed to be oxygen, assuming the sample only contains C, H, and O.
What if the sample contains no oxygen at all?
Many combustion analysis samples, such as pure hydrocarbon fuels, contain only carbon and hydrogen. In that case the mass of carbon plus the mass of hydrogen should account for essentially all of the sample mass, so subtracting them leaves a value at or near zero. This calculator treats any oxygen percentage under about 0.3% as zero and reports a formula using only C and H.
How does this differ from the general Empirical Formula Calculator?
The Empirical Formula Calculator starts from a percent composition or element masses you already know, and it supports any set of elements, not just C, H, and O. This Combustion Analysis Calculator instead performs the specific upstream conversion that produces those numbers in the first place, turning the masses of CO2 and H2O collected from an actual combustion experiment into percent C, H, and O, and then finishes with the same empirical formula step.
What atomic and molar weights does this calculator use?
It uses carbon 12.011 g/mol, hydrogen 1.008 g/mol, and oxygen 15.999 g/mol, the IUPAC 2021 standard atomic weights. From these, CO2 has a molar mass of 44.009 g/mol and H2O has a molar mass of 18.015 g/mol, which are the conversion factors used to recover element masses from the trapped combustion products.
Why does the empirical formula from combustion analysis sometimes differ from the molecular formula?
Combustion analysis only reveals the simplest whole-number ratio of atoms, the empirical formula, because it measures composition, not molecule size. Octane (C8H18) and its empirical formula C4H9 are a good example: the combustion data alone cannot distinguish octane from any other compound sharing that same 4:9 carbon-to-hydrogen ratio. A separate molar mass measurement is needed to scale up to the true molecular formula.
Can this calculator handle a sample with more than three elements?
No. This tool is built specifically for the standard CHN/CHO combustion train, which measures only carbon (via CO2) and hydrogen (via H2O), with oxygen inferred by difference. If a compound contains nitrogen, sulfur, or halogens, a standard combustion analysis cannot resolve those directly, and a more general composition tool such as the Empirical Formula Calculator, fed with independently measured element masses, is the right choice instead.
What sample mass and CO2/H2O masses should I use if I only know mass percentages?
This calculator expects the raw grams measured on a balance, not percentages. If you already have percent composition instead of the original CO2 and H2O trap masses, skip this calculator's combustion-specific conversion and use the Empirical Formula Calculator's mass percent mode directly.