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 $
