6 Abnormal Molar Masses

Abnormal Molar Masses

• Colligative property ∝   
• Due to association or dissociation of molecules, the molar mass of a substance calculated from its colligative property is either lower or higher than the expected or normal value. Such molar mass is called abnormal molar mass.

Dissociation:

KCl     K⁺ + Cl⁻

Association:

2CH₃COOH  (CH₃COOH)₂

• To account for the extent of dissociation or association, van’t Hoff introduced a factor i, known as the Van’t Hoff factor.

Value of i

For association, i < 1

For dissociation, i > 1

No association or dissociation, i = 1

• Modified equations for colligative properties after inclusion of van’t Hoff factor
• Relative lowering of vapour pressure of solvent,

• Elevation of boiling point, ΔT_b = iK_b m
• Depression of freezing point, ΔT_f = iK_f m
• Osmotic pressure of solution, π = in₂RT/V