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.

Additional recommended knowledge

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Essential Laboratory Skills Guide

The vapor pressure osmometer is comprised 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.

By measuring solutions of different concentrations of solute with a known molecular weight standard, a plot of concentration versus electrical differential can be prepared. Similarly, the unknown is then prepared and measured with the number average molecular weight derived from the standard plot.