Combustion Analysis
Definition and meaning of Combustion Analysis in chemistry.
Combustion analysis is a laboratory method for finding the empirical formula of an organic compound by burning a known mass of it completely in excess oxygen. The carbon dioxide and water produced are measured and used to calculate how much carbon and hydrogen the sample contained.
In more detail
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.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Purpose | find empirical formula |
| Method | burn sample in excess oxygen |
| Carbon becomes | CO2 |
| Hydrogen becomes | H2O |
| Carbon fraction | 12.01 / 44.01 of CO2 |
| Hydrogen fraction | 2.016 / 18.02 of H2O |
| Oxygen found | by difference |
Burning a 0.500 g sample of a compound made of only carbon, hydrogen, and oxygen produces carbon dioxide and water. The carbon mass comes from the CO2, the hydrogen mass from the H2O, and the oxygen mass by subtracting both from 0.500 g.
Frequently asked questions
Why can't oxygen be measured directly?
The sample is burned in an oxygen atmosphere, so oxygen from the air mixes with any oxygen from the compound. The compound's oxygen is instead found by subtracting the carbon and hydrogen masses from the total sample mass.
What does combustion analysis actually give you?
It gives the empirical formula, the simplest whole-number ratio of atoms. To get the molecular formula, you also need the compound's molar mass from another technique.
How is the carbon mass calculated from CO2?
Multiply the mass of carbon dioxide by 12.01 divided by 44.01, the ratio of carbon's mass to the mass of a whole CO2 molecule. This gives the mass of carbon that came from the sample.
Why must the combustion be complete?
If burning is incomplete, some carbon may form carbon monoxide or soot instead of carbon dioxide, and the measured amounts will be wrong. Excess oxygen ensures all carbon and hydrogen convert fully.