HoffFaktor

HoffFaktor, commonly known as the van't Hoff factor, is a dimensionless quantity used in chemistry to describe the number of particles produced in solution per formula unit of dissolved substance. It is denoted by i and appears in equations for colligative properties, where the effect of solute particles on properties such as boiling point, freezing point, and osmotic pressure is considered. For nonelectrolytes, i is approximately 1 because the solute does not dissociate. For electrolytes, i reflects how many particles the solute yields upon dissolution and can deviate from ideal values in real solutions.

In ideal conditions, i equals the number of particles formed from each formula unit. For example, sodium chloride (NaCl) dissociates into two ions (Na+ and Cl−), so i ≈ 2. Magnesium sulfate (MgSO4) and calcium chloride (CaCl2) can yield 2 and 3 particles respectively in idealized cases, though actual values may differ with concentration and temperature due to incomplete dissociation or ion pairing. Weak electrolytes, such as acetic acid, exhibit partial dissociation, resulting in i values lower than their theoretical particle counts. In concentrated solutions, interionic interactions often reduce the effective i below the ideal value.

HoffFaktor is applied in calculations of freezing point depression (ΔTf = i·Kf·m), boiling point elevation (ΔTb = i·Kb·m), and osmotic pressure (Π = i·M·RT). While the factor provides a practical approximation, real solutions are better described using activities rather than concentrations, especially at higher concentrations. The concept originated with Jacobus Henricus van’t Hoff in the late 19th century and remains a fundamental tool in physical chemistry.