Van 't Hoff factor

The van 't Hoff factor i (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. This is true for ideal solutions only. Sometime ion pairing occurs in solution. At a given instant a small percentage of the ions are paired and count as a single particle. Ion pairing occurs to some extent in all electrolyte solutions. This causes deviation from the van 't Hoff factor. The deviation for the van 't Hoff factor tends to be greatest where the ions have multiple charges.

Relationship with the dissociation degree

The dissociation degree is the fraction of original solute molecules that have dissociated. It is usually indicated by the Greek symbol \alpha . There is a simple relationship between this parameter and the van 't Hoff factor. If the solute substance dissociates into n ions, then

i = 1 %2B \alpha (n - 1)

For instance, for the following dissociation

KCl K+ + Cl-

As n = 2 , we would have that i = 1 %2B \alpha

Physical significance of i

The value of i is ; i = the actual number of particles in solution after dissociation รท the number of formula units initially dissolved in solution. Means the number of particles per formula unit of the solute when a solution is dilute.

See also