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Question

A very small mass of a non-volatile solute (that does not dissociate) is dissolved in 56.8cm3 of benzene (density is 0.889gcm3). At room temperature, vapour pressure of this solution is 98.88 mm Hg, while that of benzene is 100 mm Hg. Find the molality of this solution. If the freezing temperature of this solution is 0.73 degree lower than that of benzene, what is the value of molal depression constant of benzene?

A
7Kmolality1
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B
5Kmolality1
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C
3.5Kmolality1
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D
none of these
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Solution

The correct option is B 5Kmolality1
As we know,
P0PSPS=w×Mm×W
10098.8898.88=w×78×1000m×W×1000
or molality=(w×1000m×W)
PoPSPS×1000M=1.12×100078×98.88=0.1452
Also, ΔT=Kf×molality
0.73=Kf×0.1452
Kf=5.027 K molality1

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