Van't Hoff Factor
The ratio of the actual number of particles produced in solution to the number of formula units dissolved.
Formula / equation
i = observed ΔT / calculated ΔT
Unit
Dimensionless
In depth
For a non-electrolyte i equals one; for a strong electrolyte it approaches the number of ions per formula unit, though ion pairing keeps the measured value slightly lower. Solutes that associate in solution give values below one. The factor corrects every colligative equation for electrolytes.
Examples in the real world
NaCl gives i ≈ 1.9 rather than 2 because of ion pairing; benzoic acid in benzene dimerises and gives i ≈ 0.5.