Association or Dissociation of Solute Molecules

Association. In certain solvents, solute molecules bond together to form dimers, trimers, or larger aggregates, rather than remaining as separate independent molecules. Because this reduces the true number of independent particles present in solution, the molar mass calculated from a measured colligative property comes out higher than the solute's actual molar mass.

Worked example -- acetic acid in benzene. Acetic acid forms intermolecular hydrogen bonds and exists in benzene solution as a dimer (already pictured in Figure 9.9, section 9.8.3):

Its molar mass as measured via colligative properties comes out to roughly 120 g mol$^{-1}$ -- almost exactly double its true molar mass of 60 g mol$^{-1}$, consistent with two molecules effectively behaving as one particle.

Dissociation. Conversely, strong electrolytes such as KCl or NaCl dissociate completely into their constituent ions when dissolved in water. This increases the true number of particles present, so the molar mass calculated from a measured colligative property comes out lower than the actual molar mass.

Worked example -- sodium chloride in water.

$\mathrm{NaCl(s) \rightarrow Na^+(aq) + Cl^-(aq)}$ …