Question

$1 \, mol \, CH _{3} COOH$ is added in $250 \,g$ benzene. Acetic acid dimerises in benzene due to hydrogen bond, $K_{b}$ of benzene is $2 K$ kgmol $^{-1}$. The boiling point has increased by $6.4 \, K . \%$ dimerisation of acetic acid is:

• A. 50
• B. 40
• C. 30
• D. 20

Answer 1

Answer: (B)

$\Delta T _{ b }= i K _{ b } \cdot m$

Given molality $=\frac{1 \times 1000}{250}=4 m , 6.4= i \times 2 \times 4$ or $i =0.8$

For dimerisation

$i=1-\frac{\beta}{2}$

$\Rightarrow 0.8-1=\frac{-\beta}{2}$ or

$\beta=0.4 \Rightarrow 40 \%$