Combustion Analysis Calculator
Carbon from the CO2, hydrogen from the water, oxygen from whatever mass is left over.
The formula
C from the CO2, H from the H2O, O by difference
Burn it and weigh what comes off
A weighed sample is burned in excess oxygen and the carbon dioxide and water are trapped and weighed separately. All the carbon in the sample is now in the CO₂ and all the hydrogen in the H₂O, so each product mass converts straight back to an element mass. This is still how elemental analysers work.
Oxygen is never measured
It cannot be: the oxygen in the products came partly from the sample and partly from the air fed in, and nothing distinguishes them. So oxygen is taken as whatever mass is left after carbon and hydrogen are subtracted — which means every error in the other two measurements lands entirely on the oxygen figure. A sample containing no oxygen at all will still show a small positive value if the weighing is a little off.
Two routes, one answer
A 0.5000 g sample giving 0.7328 g of CO₂ and 0.3000 g of water returns CH₂O — the same formula that 40.00 / 6.71 / 53.29 percent composition gives by a completely different calculation. Agreement between the two is the usual check that neither has gone wrong.
Empirical is not molecular
Every method on these pages returns a ratio, because that is all a composition can determine. Glucose, formaldehyde and acetic acid all reduce to CH₂O, and no amount of elemental analysis will separate them. A molar mass from mass spectrometry or a freezing-point depression is what turns the ratio into a formula, and it has to come from a second, independent measurement.