Osmometry

Measure the pressure difference across a solvent-permeable barrier that retains the solute, at known temperature. For dilute polymers, the zero-concentration intercept of reduced pressure determines the number-average molar mass. Leakage, aggregation and ionic contributions can change the inferred molecular population.

For polymer mass concentration $c$, the leading ideal chain pressure is $RTc/M_n$. Plotting $\Pi /c$ against $c$ and extrapolating removes the leading virial correction. The intercept has units of energy per mass; without the measurement temperature it determines $M_n/T$, not an absolute molar mass.