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Question

75.2g of phenol is dissolved in a solvent of Kf=14. If the depression in freezing point is 7K then find the % of phenol that dimerises.

A
75%
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B
80%
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C
70%
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D
100%
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Solution

The correct option is A 75%
2C6H5OH(C6H5OH)2
Initially C 0
At equil C(1α) Cα/2

Total number of moles =CCα+(Cα/2)
=CCα2=2CCα2=C(2α)2

C=WeightMolecularweight=75.294=0.8

ΔTf=Kf×m

7=14×0.8(2α2), or 1=0.8(2α)α=0.75=75

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