What will be the value of van't Hoff factor (i) Of benzoic acid if it dimerised in aqueous solution? How will the experimental weight vary as compared to the normal molecular weight? 

In the aqueous solution, benzoic acid gets self-association because of hydrogen bonding.
The value of Van't Hoff factor is 0.5.
Molar mass of benzoic acid is 122g mol^-1. But the observed molar mass 242 g mol^-1 which is nearly double the expected molar mass, 122 g mol-1. This indicated an association of benzoic acid molecules in benzene solution into dimers.