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