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Question

An aqueous solution of urea freezes at −0.186oC. Kf for water = 1.86K kg mol−1, Kb for water = 0.512K kg mol−1. The boiling point of urea solution will be :

A
373.065 K
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B
373.186 K
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C
373.512 K
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D
373.0512 K
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Solution

The correct option is B 373.0512 K
Given that,

Kf=1.86 Kkg/mol;Kb=0.512 Kkg/mol;Tf,solution=0.186o C and Tf,water=0o C

ΔTf=0.186 K

Since,

ΔTfKf=ΔTbKb=m

ΔTb=0.1861.86×0.512=0.0512 K

As, Tb,water= 373K, and

ΔTb=0.0512 K
Since there is an elevation in boiling point upon addition of solute

Tb,solution=373+0.0512 = 373.0512 K

Hence option D is correct.

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