Solutions Question 411

Question: The vapour pressure lowering of 0.2 molal urea solution at 40°C is (assuming latent heat of vaporization $ (\Delta H) $ is 10 Kcal/mol.)

Options:

A) 0.2 torr

B) 0.1 torr

C) 0.5 torr

D) 0.3 torr

Show Answer

Answer:

Correct Answer: A

Solution:

Using Clapeyron - Clausius equation first calculate vapour pressure of water of $ \Delta H=\frac{2.303,RT_1T_2}{T_2-T_1}\log \frac{P_2}{P_1} $

$ T_1=273+100^{o}=373,K $

$ T_2=273+40^{o}=313,K $

$ 10=\frac{2.303\times 0.002\times 313\times 373}{(313-373)}\log \frac{P_2}{760} $ This gives vapour pressure $ P_2 $ of water at $ 40^{o}C=58.2,Torr $

By Raoult law $ \frac{\Delta P}{P^{o}}=\frac{w_1}{m_1}\times \frac{m_2}{w_2} $

$ \frac{w_1}{m_1}=0.2 $ mole urea $ w_2=1000,g,H_2O $

$ m=18,g/mol $

$ \Delta P=P^{o}\times \frac{w_1}{m_1}\times \frac{w_2}{m_2}=\frac{58.2\times 0.2\times 18}{1000}=0.20,Torr $



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