The method rests on a simple idea: when an organic compound burns completely, all of its carbon turns into carbon dioxide, CO2, and all of its hydrogen turns into water, H2O. By trapping and weighing these two products, a chemist can work backward to the amount of carbon and hydrogen that must have been in the original sample.
In the procedure, a weighed sample is placed in a stream of oxygen and heated so that it burns fully. The gases pass through absorbers, one that captures water and one that captures carbon dioxide. The mass gained by each absorber tells how much water and how much carbon dioxide formed.
From the CO2 mass, the mass of carbon is found using the fraction 12.01 over 44.01; from the H2O mass, the mass of hydrogen is found using the fraction 2.016 over 18.02.
Once the masses of carbon and hydrogen are known, they are converted to moles by dividing by atomic masses. If the compound contains only carbon, hydrogen, and oxygen, the mass of oxygen is found by subtracting the carbon and hydrogen masses from the total sample mass. The oxygen cannot be measured directly because the burning takes place in an oxygen atmosphere.
The final step is to turn the mole amounts into a whole-number ratio, which gives the empirical formula. To get the molecular formula, the chemist also needs the compound’s molar mass, found by another method such as mass spectrometry. Dividing the molar mass by the empirical formula mass shows how many empirical units make up one molecule.
Combustion analysis was a cornerstone of early organic chemistry and is still used to check the purity and identity of new compounds.