Vont hoff factor| Its relation with degree of dissociation and association

The van't Hoff factor I for an electrolyte which undergoes dissociation and association in solvent place as follows

Define van't Hoff factor . How is it related to the degree of dissociation ?

Acetic acid undergoes dimerisation in benzene, the van't Hoff factor i is related to degree of association of acid a, as