The molar mass of ethanoic acid found by the freezing-point depression method in benzene is about $120\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$, twice its formula mass. This is because ethanoic acid in benzene

In a non-polar solvent the acid molecules pair through two O-H...O hydrogen bonds into a cyclic dimer. The number of solute particles is halved, so the colligative effect is halved and the apparent molar mass doubles to about $2\times 60=120$. Ionisation would increase the particle count and lower the apparent molar mass.