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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 mol−1]

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 mThe depression if the freezing point ΔTf=i⋅Kf⋅mΔTf=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 constantm= molalityΔTf=i⋅Kf⋅m10 K=1×1.86 Kkg/mol×mm=5.3 mol/kg=5.3 mHence, the molality of solution is 5.3 m.

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