Question

# The freezing point of water is depressed by 0.37∘C in a 0.01 molal NaCl solution. The freezing point of 0.02 molal solution of the urea is depressed by:

A
0.37C
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B
0.74C
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C
0.185C
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D
0C
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Solution

## The correct option is A 0.37∘CAs we know that,Δtf=ikfmwhere, Δtf = depression in freezing pointi=van't - Hoff factorm=molarityand kf=freezing point depression constant.For 0.01 molal NaCl solution0.37=2×kf×0.01∴kf=0.372×0.01....(i)For 0.02 molal urea solutionΔtf=1×kf×0.02∴kf=Δtf0.02...(ii)From Eqs (i) and (ii)0.372×0.01=Δtf0.02Δtf=0.37×0.022×0.01∴tf=0.37∘CHence, the correct option is A

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