Solutions Question 69
Question: A solution of urea $ ( mol.mass56gmo{l^{-1}} ) $ boils at 100.18°C at the atmospheric pressure. If $ K_{f} $ and $ K_{b} $ for water are 1.86 and $ 0.512Kkgmo{l^{-1}} $ respectively the above solution will freeze at [CBSE PMT 2005]
Options:
A) $ ,- 6.54{}^\circ C $
B) $ 6.54{}^\circ C $
C) $ 0.654{}^\circ C $
D) $ -0.654{}^\circ C $
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Answer:
Correct Answer: D
Solution:
$ \Delta T_{b}=0.18 $ ; $ \Delta T_{b}=mK_{b} $
$ \frac{0.18}{\Delta T_{f}}=\frac{mK_{b}}{m,K_{f}} $ ; $ \frac{0.18\times 1.86}{0.512}=\Delta T_{f} $ ; $ \Delta T_{f}=0.653 $
$ T^{0}-T_{s}=0.653 $ ; $ T^{0}-T_{s}=0.653 $ ; $ T_{s}=0-{{0.653}^{o}}C $ .
