Determine the number of particles produced by one formula unit of solute in solution
Use the formula i = actual number of particles in solution / number of formula units dissolved
For nonelectrolytes, i = 1
For strong electrolytes, i equals the total ions formed per formula unit
For weak electrolytes, i is less than the theoretical value because of incomplete dissociation
For association, i is less than 1
For dissociation, i is greater than 1
If using colligative properties, calculate i from:
i = observed colligative property / calculated colligative property assuming no dissociation or association
For boiling point elevation, i = ΔTb(observed) / ΔTb(calculated)
For freezing point depression, i = ΔTf(observed) / ΔTf(calculated)
For osmotic pressure, i = π(observed) / π(calculated)
For relative lowering of vapor pressure, i = observed value / calculated value
For electrolytes with degree of dissociation α, use i = 1 + α(n – 1)
In the formula, n = number of ions formed per formula unit on complete dissociation
For association with degree of association α and m molecules associating, use i = 1 – α(m – 1)