Van 't Hoff factor

The van 't Hoff factor i (named after Dutch chemist Jacobus Henricus van 't Hoff) is a measure of the effect of a solute on colligative properties such as osmotic pressure, relative lowering in vapor pressure, boiling-point elevation and freezing-point depression.

For most non-electrolytes dissolved in water, the van 't Hoff factor is essentially 1.

For most ionic compounds dissolved in water, the van 't Hoff factor is equal to the number of discrete ions in a formula unit of the substance.

At a given instant a small percentage of the ions are paired and count as a single particle.

This causes the measured van 't Hoff factor to be less than that predicted in an ideal solution.

The deviation for the van 't Hoff factor tends to be greatest where the ions have multiple charges.

There is a simple relationship between this parameter and the van 't Hoff factor.

For dissociation in the absence of association, the van 't Hoff factor is:

), then For the dimerisation of acetic acid in benzene: 2 moles of acetic acid associate to form 1 mole of dimer, so that For association in the absence of dissociation, the van 't Hoff factor is: