# Method according to Victor Meyer

historical apparatus by Victor Meyer

By the method of Victor Meyer which can molar mass of a vaporizable liquid be determined. Today, however, the method has more of a didactic benefit for training chemists or teaching chemistry .

## Basics

A certain amount of a liquid is vaporized. Using the general gas equation for ideal gases , the molar mass can be calculated from the volume of the evaporated liquid.

${\ displaystyle p \ cdot V = n \ cdot R \ cdot T}$
${\ displaystyle p \ cdot V = {\ frac {m} {M}} \ cdot R \ cdot T}$

If this equation is solved for the molar mass M , then:

${\ displaystyle M = {\ frac {m \ cdot R \ cdot T} {p \ cdot V}}}$

Here m is the weighed-in mass of the substance, R the general gas constant , T the temperature of the gas volume, p the pressure of the gas volume and V the volume of the gas.
With the temperature T in Kelvin , the pressure p in Pascal and the volume V in m 3 . The mass m can be given in g or kg, accordingly the unit for the molar mass M is g / mol or kg / mol. ${\ displaystyle R = 8 {,} 314472 \; {\ frac {\ mathrm {J}} {\ mathrm {mol \, K}}}}$

## Procedure

The apparatus according to Victor Meyer consists of a narrow evaporation tube, the extended part of which is located in a heating bath (water for substances that boil below 100 o C). The evaporation tube is also surrounded by a heating jacket. At the top, the apparatus is connected to a device ( gas burette with level compensation or piston sampler ) for collecting the gas. The substance to be evaporated is weighed precisely in a substance vial and placed in the evaporation tube, after which the apparatus is immediately closed again. The volume of the vaporized substance now displaces a corresponding volume of air, which is determined with the aid of the collecting device. The temperature to be determined is that of the captured air volume, usually the ambient temperature, and not that in the evaporation pipe. After pressure equalization, the pressure is the prevailing air pressure.

In a simplified method for easily evaporable liquids that evaporate at room temperature, such as B. diethyl ether , the heating bath is omitted and a large flask is used instead of the long evaporation tube.

In another variant, an electrically heated piston sampler can be used. The substance is precisely weighed and brought into the interior of the gas syringe and evaporated. For evaluation, the increase in volume of the air previously present in the gas syringe must be subtracted from the increase in volume. The temperature is measured in the gas syringe.

## literature

• Gottwald / Puff: Physikalisch-Chemisches Praktikum, VCH Weinheim 1987, p. 29 ff, ISBN 3-527-26498-1
• H. Böhland: Chemische Schulexperimente Volume 5: General, physical and analytical chemistry, Verlag Harri Deutsch Thun - Frankfurt 1979, p. 25 ff, ISBN 3-87144-358-1