Question

Molecules of benzoic acid (C$_{6}H_{5}$COOH) dimerise in benzene. $'w' \,g$ of the acid dissolved in $30\, g$ of benzene shows a depression in freezing point equal to $2K$. If the percentage association of the acid to form dimer in the solution is $80$, then $w$ is : (Given that $K_{f} = 5 K\, kg\, mol^{-1}$, Molar mass of benzoic acid $= 122 \,g\, mol^{-1})$

• A. 1.8 g
• B. 2.4 g
• C. 1.0 g
• D. 1.5 g

Answer 1

Answer: (B)

2.4 g

Answer 2

{ 2 \underset{Wg}(C_6H_5 COOH) ->[\text{C_6H_6}][\text{(30 g)}] \underset{diner}(C_6H_5 COOH)_2} $\Delta_1 T \, \, = \, i \, k_r \, m$ $2 = 0.6 \times \, 5 \, \times \, \, \frac{w \times 1000}{122 \times 30}$ $(i = 1 - 0.8 +0.4 = 0.6)$ $w = 2.44\, g$