Molar mass determination

This can be done quickly using mass spectrometry. However, this is beyond the ability of most school and college laboratories. Here the most likely method of molar mass determination is by measuring the volume of vapour produced from a known mass of liquid. A sample of liquid is weighed in a syringe and this is then injected through a septum into a heated gas syringe. The liquid vaporizes and the volume of vapour is measured along with the temperature. The syringe is reweighed to give the mass of liquid used.

In order to calculate the molar mass you will need to know about the ideal gas equation.

Molar mass can also be determined experimentally by depression of freezing point. The presence of impunities lowers the melting point of a pure solid. This is the reason that salt is added to roads in the winter - the ice melts at below 0 ºC. There is a mathematical link between the number of moles of impurity present and the lowering of the freezing point. Knowing the mass of substance added as an impurity and the change in melting point, it is possible to work out the molar mass.

A pile of grit (salt and sand) by a roadside for lowering the melting point of ice