Hofffaktorn

Hofffaktorn, often referred to as the van’t Hoff factor and denoted i, is a dimensionless quantity in physical chemistry that describes the number of particles a solute produces in solution per formula unit. It is a key factor in relating colligative properties to solute concentration, such as freezing point depression, boiling point elevation and osmotic pressure.

In an ideal solution, i reflects the extent of dissociation or association of the solute. For strong electrolytes that dissociate completely, i equals the number of particles formed. For example, NaCl yields two ions (Na+ and Cl−), so i is about 2 at infinite dilution; MgCl2 yields three ions (Mg2+ and two Cl−), so i is about 3. For non-electrolytes that do not dissociate, i is 1. For weak electrolytes, i lies between 1 and the maximum number of particles produced by complete dissociation and depends on the degree of dissociation α.

Quantitatively, colligative properties depend on i. Freezing point depression follows ΔTf = i Kf m, boiling point elevation follows ΔTb = i Kb m, and osmotic pressure follows Π = i M R T, where m is molality, M is molarity, and Kf, Kb, R and T have their usual meanings. In practice, i can deviate from ideal values due to ion pairing, solvent interactions, and finite concentration; activity coefficients and effective dissociation fractions must be considered at higher concentrations or non-ideal conditions.

The concept is widely used in chemistry and chemical engineering, including electrolyte solutions, battery electrolytes, and analytical chemistry, to interpret how solutes influence colligative properties and to estimate dissociation behavior.