Membrane osmometer
![The operating principle of a membrane osmometer. Water (below) is connected to the solution to be measured (above) via a membrane that lets water through.](/Images/godic/202501/26/Osmometer.svg5548.png")
A membrane osmometer is a device used to indirectly measure the number average molecular weight (![M_n](/Images/godic/202501/26/1547a74739c1e2b6be3ebe927017bfbb5548.png")
) of a polymer sample. One chamber contains pure solvent and the other chamber contains a solution in which the solute is a polymer with an unknown . The osmotic pressure of the solvent across the semipermeable membrane is measured by the membrane osmometer. This osmotic pressure measurement is used to calculate for the sample.