Molecules of benzoic acid C 6 H 5 COOH dimerise in 30   g of benzene. ' w ' g of benzoic acid shows a depression in freezing point equal to 2 K . If the percentage association of the acid to form dimer in the solution is 80 , then w is: ( Given that K f = 5 Kmol - 1 , molar mass of benzoic acid = 122 gmol - 1 )

Step-by-step solution

ΔT f   = k f × m × i … ( i ) Molecular weight of C 6 H 5 COOH = 112 n = 2 because benzoic acid is dimerised, β = 80 % = 0.8 Here i = 1 n - 1 β + 1 i = 1 2 - 1   0.8 + 1 = 0.6 Put the value in equation ( i ) 2=0 .6× wt . of benzoic acid 112 30 1000 ×5 weight of benzoic acid = 2.4  g