Question

18 g of glucose is dissolved in 178.2 g of water. The vapour pressure of the solution at ${100}^{o}C$ is :

(vapour pressure of pure water at ${100}^{o}C$ is 760 mm Hg)

• A. 767.6 mm Hg
• B. 760 mm Hg
• C. 752.4 mm Hg
• D. 725.4 mm Hg
• E. 745.2 mm Hg

Answer 1

The correct option is C 752.4 mm Hg

Vapour pressure $\left(P^o\right)$ of pure water at ${100}^{o}C$ is = 760 mm Hg

From relative lowering of vapour pressure :

$\frac{p^o-p_s}{p^o}=\frac{w_2}{m_2}\times \frac{m_1}{w_1}$

$\frac{760-p_s}{760}=\frac{18}{180}\times \frac{18}{178.2}=\frac{1}{99}$

$760-p_s=\frac{760}{99}$

or $760-p_s=7.6$

$\Rightarrow p_s=752.4mmHg$