What is the van 't Hoff factor?
The number of particles a dissolved substance actually makes, divided by what you'd expect if it stayed whole. Sugar gives i ≈ 1 (no split), table salt i ≈ 2 (Na⁺ + Cl⁻), calcium chloride i ≈ 3.
// chemistry › Dissociation
van 't Hoff Factor (i)
Calculate the van 't Hoff factor i = observed/calculated: particles per dissolved unit, with symbol legend and real-world examples.
i = \frac{\text{observed effect}}{\text{calculated effect}}
A mind behind this: Jacobus Henricus van 't Hoff 1852–1911
Frequently asked questions
Why does it matter?
Because freezing-point drop, boiling-point rise, and osmotic pressure all depend on the number of particles. A salt that splits in two affects them about twice as much as a non-splitting solute - which is why salt melts ice so well.
Why is the real factor often a bit less than the whole number?
In real solutions some separated ions briefly pair up again, so the effective particle count is slightly below the ideal. NaCl's ideal i is 2 but measured values are around 1.8-1.9.
How does it connect observed and calculated?
i = observed ÷ calculated. If a salt drops the freezing point twice as much as a non-splitting solute would, the observed is double the calculated, so i = 2. Measuring an effect reveals how many particles formed.
Can it tell us what a solute does in solution?
Yes - i above 1 means it splits into ions, i below 1 means particles join together (association). So measuring i reveals the actual behaviour of the dissolved substance.