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Question

What is the molality of C2H5OH in water solution which will freeze at 10oC? [Mol. wt. of C2H5OH=46,Kf for water =1.86 K kg mol1]

A
6.315 m
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B
63.15 m
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C
3.540 m
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D
5.3 m
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Solution

The correct option is D 5.3 m
The depression if the freezing point
ΔTf=iKfm
ΔTf=10 0C=10 K= the depression if the freezing point
i=1= van't Hoff's factor (ethanol is non electrolyte and does not dissociate in aqueous solution).
Kf=1.86 Kkg/mol= molal depression in freezing point constant
m= molality
ΔTf=iKfm
10 K=1×1.86 Kkg/mol×m
m=5.3 mol/kg=5.3 m
Hence, the molality of solution is 5.3 m.

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