Question

1. experimentally, i ( the van't Hoff factor) is usually less than the ideal value due to the formation of ion pairs. What would the formation of ion pairs do to the concentration of solute in solution? what is the effect of ion pairing on Change in Tf

Answer 1

Ion pairs are formed when two oppositely charged ions are very close to one another due to strong electrostatic forces of attraction with no solvent in between. An ion-pair counts as single particle and hence ion-pair formation would decrease the concentration of free ions from the solute in solution. As the depression in freezing point is dependent on the number of particles in a solution, a reduced number of particles due to ion-pair formation will lead to a reduction in the change in Tf I.e. a smaller depression in freezing point.