Dumas Method
Definition and meaning of Dumas Method in chemistry.
Dumas method is a classical technique for determining the molar mass of a volatile liquid by vaporizing a small sample in a flask of known volume and measuring the mass of vapor that fills it at a known temperature and pressure.
In more detail
A weighed liquid sample is placed in a bulb fitted with a fine pinhole and heated in a bath above the liquid's boiling point; excess vapor escapes through the pinhole until only vapor at bath temperature and atmospheric pressure remains inside. The bulb is then sealed, cooled, and reweighed to find the mass of vapor trapped, and the ideal gas law (PV = nRT) converts this into moles and hence molar mass. Because the method assumes ideal gas behavior and requires the compound to vaporize cleanly without decomposing, it works best for stable, moderately volatile liquids and gives approximate rather than highly precise results.
Key facts
| Field | Physical Chemistry |
|---|---|
| Developed by | Jean-Baptiste Dumas (1826) |
| Key equation | M = mRT / PV |
| Used for | Molar mass of volatile liquids |
To find the molar mass of chloroform, a few milliliters are sealed in a tared glass bulb with a pinhole, heated in a boiling water bath (100°C) until only vapor remains, then cooled and weighed; applying M = mRT/PV to the mass gained yields a value close to 119 g/mol.
Frequently asked questions
What are the main limitations of the Dumas method?
It assumes ideal gas behavior, requires the sample to vaporize without decomposing, and is less precise than modern methods like mass spectrometry.
How does the Dumas method differ from the Victor Meyer method?
The Dumas method weighs the vapor directly, while the Victor Meyer method measures the volume of air displaced by the vapor, often giving more reproducible results.