Abnormal Molar Mass

The molar mass of a nonvolatile solute can, in principle, be calculated accurately from any of the four colligative properties (sections 9.9.1-9.9.4). But every one of those calculations rests on an assumption: that the solution is very dilute, and that the solute neither associates nor dissociates once dissolved.

In a concentrated solution, the interaction between solute and solvent becomes significant enough to be reflected directly in the measured colligative property, throwing off the calculation. Similarly, if the dissolved solute molecules associate (combine together) or dissociate (split apart) in solution, the true number of particles present differs from the number of formula units originally dissolved -- and since every colligative property depends on the number of particles, this too skews the measured value away from what a simple, non-associating/non-dissociating solute would give. …