Question

2 g of benzoic acid dissolved in 25 g of $C_6{H}_6$ shows a depression in f.pt. equal to 1.62 K. $K_f$ for $C_6{H}_6$ is 439 K $molalit{y}^{-1}$. The percentage association of acid, if it forms double molecules in solution is

• A. $0.8$ %
• B. $99.2$ %
• C. $90.2$ %
• D. $9.8$ %

Answer 1

Answer: (B)

$99.2$ %

The percentage association of acid in the solution is 99.2 %
$\Delta T=\frac{1000\times K_f\times w}{m\times W}=\frac{1000\times 4.9\times 2}{122\times 25}=3.213$

$i=\frac{\Delta T_{exp}}{\Delta T_{cal}}=1-\alpha +\frac{\alpha }{n}$

$\therefore \frac{1.62}{3.213}=1-\alpha +\frac{\alpha }{2}$

$\therefore \alpha =0.992$ or 99.2 %