Question

Elevation in boiling point of an aqueous urea solution is $ 0.52{}^\circ C[{{K}_{b}}=0.52{}^\circ mo{{l}^{-1}}kg]. $ Hence, mole fraction of urea in this solution is

• A. 0.982
• B. 0.567
• C. 0.943
• D. 0.018

Answer 1

Answer: (D)

$ \because $ $ \Delta {{T}_{b}}=molality\times {{K}_{b}} $
$ \because $ $ 0.52=m\times 0.52 $
or molality $ =1\text{ }mol\text{ }k{{g}^{-1}} $
$ \therefore $ No. of moles of urea = 1 mol No. of moles of water $
=\frac{1000}{18}=55.55\text{ }mol $
Mole fraction of urea $ =\frac{moles\text{ }of\text{ }urea}{moles\text{ }of\text{ }urea\text{ }+\text{ }moles\text{ }of\text{ }water}=0.018 $
$ =\frac{1}{1+55.55} $
$ =\frac{1}{56.55} $