Phenol associates in benzene to certain extent to form a dimer. A solution that is containing 20 x 10-3kg of phenol dissolved in 1 kg of benzene has its freezing point decreased by 0.61 K. Calculate the fraction of phenol that has dimerised.
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∆ Tf = Kf wB 1000/ wA MB
0.69 = 5.12 x 20 x 10^-3 x 10^3 / 1 x MB
MB = 5.12 x 20 / 1 x 0.69
Molecular weight of phenol = 94g
Calculated M . W / observed M . W = No. of particles after association/ No. of particles before association
94 / 148.4 = 1 - x/2 / 1
x = 0.73 per mole
Fraction of C6H5OH = 0.73 x 100 = 73%
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