Solutions Question 45
Question: A solution containing 30 gms of non-volatile solute in exactly 90 gm water has a vapour pressure of 21.85 mm Hg at $ 25^{o}C $ . Further 18 gms of water is then added to the solution. The resulting solution has a vapour pressure of 22.15 mm Hg at $ 25^{o}C $ . Calculate the molecular weight of the solute [UPSEAT 2001]
Options:
A) 74.2
B) 75.6
C) 67.83
D) 78.7
Show Answer
Answer:
Correct Answer: C
Solution:
We have, $ \frac{p^{0}-21.85}{21.85}=\frac{30\times 18}{90\times m} $ , for I case …..(i) wt. of solvent $ =90+18=108gm $
$ \frac{p^{0}-22.15}{22.15}=\frac{30\times 18}{108\times m} $ , for II case …..(ii) By eq. (1) $ p_m^{0}-21.85m=21.85\times 6=131.1 $ By eq. (2) $ p_m^{0}-22.15m=22.15\times 5=110.75 $ 0.30m = 20.35 $ m=\frac{20.35}{0.30}=67.83 $
