Bunsen method

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The method according to Bunsen (after Robert Wilhelm Bunsen ) is a method with which the molar mass of a gas can be determined via the outflow time of the gas.


According to Graham's law, the following applies to the outflow of gases:

The following applies to the outflow times:

So if one knows the outflow times of two gases and the molar mass of one gas, then the molar mass of the other gas can be calculated according to


Apparatus for determining the outflow times of gases

As a modification to the original Bunsen effusiometer, in the training area to illustrate the relationships, the determination can be made with the aid of a vertically attached piston sampler . The piston syringe has a stopcock at the bottom and a glass tube which is closed with a nozzle. The gas syringe is first filled with a known gas and the time it takes for it to flow out is determined. One proceeds accordingly with the unknown gas.

For 100 ml of an unknown hydrocarbon compound, the outflow time t 2 = 9.0 seconds and for 100 ml nitrogen t 1 = 8.8 seconds. The molar mass for nitrogen is M 1 = 28 g / mol.
It could be ethane .


  • R. Brdička: Fundamentals of physical chemistry, VEB Deutscher Verlag der Wissenschaften Berlin 1967, p. 207
  • Böse / Schmidt: Quantitative experiments with the piston sampler, Diesterweg Salle Frankfurt 1972, p. 26, ISBN 3-425-05076-1