Van 't Hoff factor

This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2009)

The van 't Hoff factor (named after J. H. van't Hoff) is a measure of the effect of a solute upon colligative properties, such as vapor pressure, osmotic pressure and freezing point depression. The van 't Hoff factor is the ratio between the actual concentration of particles produced when the substance is dissolved, and the concentration of a substance as calculated from its mass. 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.

Physical significance of i

• When solute particles associate in solution, i is less than unity. [Eg. Ethanoic acid in benzene, benzoic acid in benzene]
• When solute particles dissociate in solution, i is greater than unity. [Eg. Sodium chloride in water, potassium chloride in water, magnesium chloride in water]
• When solute particles neither dissociate nor associate in solution, i equals unity. [Eg. Glucose in water]

Relation to degree of dissociation

• When a solute dissociates in solution,

• When a solute associates in solution,

Where α is the degree of association/ dissociation and n is the number of particles produced during association or dissociation per molecule or formula unit of the solute.

Illustrative examples

• For NaCl in water, which dissociates completely into Na⁺ and Cl^- ions in solution, α equals 1 (100%) and n equals two (since two ions, namely one Na⁺ ion and one Cl^- ion are produced in solution per formula unit of NaCl). van't Hoff factor for NaCl is then calculated as 2 which means that the true formula weight of NaCl is one half that calculated by colligative methods ignoring the van't Hoff factor.

See also

• Colligative properties
• Solutions
• Raoult's law
• Van 't Hoff equation