Abnormal molar mass
Abnormal molar mass
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Some molecules of solvents with low dielectric constant like molecules of ethanoic acid dimerize into benzene due to the presence of hydrogen bond.
Association of all the molecules of ethanoic acid into benzene means Tb or Tf for ethanoic acid will be half of the normal value.
The molar mass calculated on the basis of this ΔTb or ΔTf will be twice the expected molar mass.
This value of molar mass is either lower or higher compared to expected or normal value and is known as abnormal molar mass.
In 1880 van’ t Hoff introduced a factor i and was named as the van’ t Hoff factor.
The van’ t Hoff factor account for the degree of dissociation or association.
The van’ t Hoff factor i can be defined as the ratio of Normal molar mass to the abnormal molar mass. Mathematically,
o In case of association, value of i is less than unity. o In case of dissociation value of i is greater than unity. o Relative lowering of vapour pressure of solvent =
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Elevation of Boiling point, ΔTb = i Kb m Depression of Freezing point, ΔTf = i Kf m Osmotic pressure of solution, ð = i n2RT / V