The van't Hoff factor ($i$) is the ratio of the total number of moles of particles after association/dissociation to the number of moles of particles before association/dissociation. When a solute undergoes dissociation in a solvent, the number of particles in the solution increases. Therefore, the value of the van't Hoff factor ($i$) is greater than one. Conversely, when a solute undergoes association in a solvent, the number of particles in the solution decreases. Therefore, the value of the van't Hoff factor ($i$) is less than one.