Question

The freezing point of a $1.00\, m$ aqueous solution of HF is found to be $-1.91 ^{\circ}$C. The freezing point constant of water, $K_f$ is $1.86 \,K \,kg \,mol^{-1}$. The percentage dissociation of HF at this concentration is

• A. 30%
• B. 10%
• C. 5.2%
• D. 2.7%

Answer 1

Answer: (D)

$\Delta T_{f} = K_{f} \times m \times i$
$i = \frac{\Delta T_{f}}{K_{f} \times m} = \frac{1.91}{1.86 \times1} = 1.02$
For $\underset{\left(1-\alpha\right)}{HI }\rightleftharpoons \underset{\alpha}{H^+ }+ \underset{\alpha}{I^- }$
$1-\alpha +\alpha+\alpha = i = 1.027$
$1+\alpha = 1.027$
$\alpha = 0.027$ or $2.7\%$