Solutions Question 250

Question: What mass of non-volatile solute (urea) needs to be dissolved in 150 g of water in order to decrease the vapour pressure of water by 35%. What will be the molality of solution'

Options:

A) 23.88

B) 31.12

C) 25.47

D) 29.92

Show Answer

Answer:

Correct Answer: D

Solution:

Using Raoult’s Law: $ \frac{P^{0}-P_{S}}{P_{S}}=\frac{W_2Mw_1}{Mw_2.W_1} $ If $ P^{0}=100mm $ then $ P_{S}=100-35=65,mm $

$ \frac{100-65}{65}=\frac{W_2\times 18}{60\times 18} $
$ \Rightarrow W_2=\frac{35\times 60\times 150}{65\times 18}=269.23 $
$ \therefore $ Molality $ =\frac{W_2\times 1000}{Mw_2\times W_1}=\frac{269.23\times 1000}{60\times 150} $

$ =29.92m $



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