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Question

A solution of urea (mol. mass 56 gmol1) boils at 100.18oC at the atmospheric pressure. If Kf and Kb for water are 1.86 and 0.52 Kkg.mol1 respectively, the above solution will freeze at:

A
6.54o
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B
6.54oC
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C
0.654oC
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D
0.654oC
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Solution

The correct option is D 0.654oC
ΔTf=Kf× molality of solution

and ΔTb=Kb× molality of solution

ΔTfΔTb=KfKb

Given that

ΔTb=T2T1=100.18100=0.18oC

Kf for water = 1.86 K kg mol1

Kb for water = 0.52 K log mol1

ΔTf0.18=1.860.512

or ΔTf=1.86×0.180.512

=0.65390.65

ΔTf=T1T2

0.654=0oCT2

T2=0654oC

(T2 Freezing point of aqueous urea solution).

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