Benzoic acid (true molar mass 122 g/mol) dimerises in benzene and shows an observed molar mass of 242 g/mol. The percentage association of the acid is approximately:

1. $i = $ normal / observed molar mass = $122 / 242 = 0.504$. 2. For dimerisation, total moles per starting mole = $1 - x/2$, so $i = 1 - x/2$. 3. $x/2 = 1 - 0.504 = 0.496$, hence $x = 0.992$. 4. Percentage association = $99.2\%$. (Option A is x/2, not x.) _Source: NCERT Class 12 Chemistry Ch 1 "Solutions", p.25_