# Abnormal Molecular masses Chemistry Notes

## Abnormal Molecular masses Chemistry Notes

The measurement of correct molecular mass can take place with the help of colligative properties only when

• Solution is dilute
• Solution obeys Raoult’s law
• Solute is neither dissociated nor associated in solution.

→ If dissociation or association of solute takes place in dilute solution then the value of molecular mass calculated with the help of colligative properties is different from actual molecular mass. This molecualar mass is called abnormal molecular mass. For example, 1 mole of NaCl, CaCl2, K4 [Fe(CN)6) is dissolved in water and ionization is 100%, then their one mole forms 2, 3, 5 mole ions respectively.

→ Since, colligative properties are inversely proportional to molecular mass. So, the molecular mass of these compounds are obtained as $$\frac{1}{2}, \frac{1}{3}, \frac{1}{5}$$ parts respectively from actual molecular mass.

→ Similarly, formic acid, acetic acid and benzoic acid from dimer in benzene solvent which are associated by hydrogen bond.

→ Dimer of Acetic acid : Since colligative properties are inversly proportional to molecular mass. So, the molecular masses of above carboxylic acids are obtained twice from actual molecular mass.