Question

What is the freezing point of a solution containing $8.1gHBr$ in $100g$ water assuming the acid to be 90% ionised $(K_{f}$ for water $= 1.86Kmole^{- 1})$.

• A. $0.85^{o}C$
• B. $- 3.53^{o}C$
• C. $0^{o}C$
• D. $- 0.35^{o}C$

Answer 1

Answer: (B)

$- 3.53^{o}C$

Answer 2

$\underset{(1 - \alpha)}{HBr}$ ⇌ $\underset{\alpha}{H^{+}} + \underset{\alpha}{Br^{-}}$

Total = 1 + α ∴ $i = 1 + \alpha = 1 + 0.9 = 1.9$

$\Delta T_{f} = iK_{f} \times m = 1.9 \times 1.86 \times \frac{8.1}{81} \times \frac{1000}{100} = 3.53^{o}C$

$T_{f} = - 3.53^{o} ⥂ C$.