# Bunsen method

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.

## Basics

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

${\ displaystyle {v_ {1} \ over v_ {2}} = {\ sqrt {M_ {2} \ over M_ {1}}}}$

The following applies to the outflow times:

${\ displaystyle {t_ {2} \ over t_ {1}} = {\ sqrt {M_ {2} \ over M_ {1}}}}$

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

${\ displaystyle M_ {2} = {\ frac {t_ {2} ^ {2} \ cdot M_ {1}} {t_ {1} ^ {2}}}}$

## Procedure

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.

example
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.
{\ displaystyle {\ begin {aligned} M_ {2} & = {\ frac {(9 {,} 0 \, \ mathrm {s}) ^ {2} \ cdot 28 \, \ mathrm {g} / \ mathrm {mol}} {(8 {,} 8 \, \ mathrm {s}) ^ {2}}} \\ & = 29 {,} 3 \, \ mathrm {g} / \ mathrm {mol} \ end { aligned}}}
It could be ethane .

## literature

• 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