Pure benzene freezes at 5.3∘C. A solution of 0.223g of phenylacetic acid (C6H5CH2COOH) in 4.4g of benzene (Kf=5.12Kkg mol−1) freezes at 4.47∘C. From the observation, one can conclude that
A
Phenylacetic acid exists as it is in benzene
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B
Phenylacetic acid undergoes partial ionisation in benzene
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C
Phenylacetic acid undergoes complete ionisation in benzene
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D
Phenylacetic acid dimerises in benzene
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Solution
The correct option is D Phenylacetic acid dimerises in benzene ΔTf=T0−Tf=5.3−4.47=0.83
Molality of solvent, m=0.223136×10004.4=0.373 (Molecular weight of phenyl acetic acid=136gmol−1 Kf=5.12 ΔTf=i.Kfm i=(ΔTf)/(Kfm)=0.835.12×0.373≈0.43
It is nearly equal to 0.5 which means that the phenyl acetic acid undergoes dimerization in benzene.