Vapor pressure osmometry

Vapor pressure osmometry is a technique to measure the number average molecular weight of a polymer. It is based upon Raoult's law that governs change in vapor pressure of a solution based on the mole fraction of the solute.

The vapor pressure osmometer is composed of two chambers: one for pure solvent and the other to contain solution, where the solute is the polymer whose molecular weight is unknown. Thermistors in each chamber provide an electrical signal (the actual measurement) of differential heating to achieve a vapor equilibrium in each chamber.