Molecules of benzoic acid C 6 H 5 COOH dimerise in 30   g of benzene. ' w ' g of benzoic acid…

Molecules of benzoic acid C6H5COOH 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 Kf=5 Kmol-1, molar mass of benzoic acid =122 gmol-1)
  1. 1.0 g
  2. 2.4 g
  3. 1.8 g
  4. 1.5 g

Solution

ΔTf =kf×m×i …(i)

Molecular weight of C6H5COOH=112

n = 2  because benzoic acid is dimerised, β = 80 % = 0.8  

Here i=1n-1β+1

i=12-1 0.8+1

=0.6

Put the value in equation (i)

2=0.6×wt.of benzoic acid112301000×5

weight  of benzoic acid =2.4 g

Asked in: JEE Main 2019 (12 Jan Shift 2)

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