Abnormal molar mass
- 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,
- In case of association, value of i is less than unity.
- In case of dissociation value of i is greater than unity.
- Relative lowering of vapour pressure of solvent =
- Elevation of Boiling point, ΔTb = i Kb mDepression of Freezing point, ΔTf = i Kf mOsmotic pressure of solution, ð = i n2RT/V
