AP | A-Level | IB | NCERT 11 + 12 – FREE NOTES, RESOURCES AND VIDEOS!
1 Solutions 2 Electrochemistry 3 Chemical Kinetics 4 The d-and f-Block Elements 5 Coordination Compounds 6 Haloalkanes and Haloarenes 7 Alcohols, Phenols and Ethers 8 Aldehydes, Ketones and Carboxylic Acids 9 Amines 10 Biomolecules

1 Solutions

1.1 Types of Solutions 1.2 Expressing Concentration of Solutions 1.3 Solubility 1.4 Vapour Pressure of Liquid Solutions 1.5 Ideal and Non-ideal Solutions 1.6 Colligative Properties and Determination of Molar Mass 1.7 Abnormal Molar Masses 2.1 Electrochemical Cells 2.2 Galvanic Cells 2.3 Nernst Equation 2.4 Conductance of Electrolytic Solutions 2.5 Electrolytic Cells and Electrolysis 2.6 Batteries 2.7 Fuel Cells 2.8 Corrosion

Abnormal Molar Masses

NCERT Reference: Chapter 1 – Solutions – Page 24

Quick Notes

  • Some solutes show abnormal molar masses when determined via colligative properties.
  • Abnormality arises due to either:
    • Association of solute molecules (e.g. acetic acid in benzene),
    • Dissociation of solute into ions (e.g. KCl in water).
  • The van’t Hoff factor (i) is introduced to account for this abnormality.
  • Modified formula for molar mass: Observed molar mass = Normal molar mass / i
  • Colligative property expressions are corrected by multiplying with van’t Hoff factor (i).
  • If i > 1 → dissociation (e.g. salts),
    If i < 1 → association (e.g. carboxylic acids).

Full Notes

We can use colligative properties (like boiling point elevation or freezing point depression) to find the molar mass of a non-volatile solute. But sometimes, we get unusual (abnormal) results. This happens when solute particles join together (association) or split apart (dissociation) in solution.

Colligative Properties

Association or dissociation changes the number of solute particles in solution, which directly affects colligative properties.

Example: Association in non-aqueous solvent

Acetic acid in benzene forms dimers: 2 CH3COOH ⇌ (CH3COOH)2.
Fewer particles means smaller effect on properties and apparent molar mass appears higher.

NCERT 12 Chemistry illustration showing association of acetic acid into a hydrogen-bonded dimer in benzene causing fewer particles and higher apparent molar mass.

Example: Dissociation in aqueous solution

Potassium chloride (KCl) in water dissociates: KCl → K+ + Cl.
More particles means bigger effect on properties and apparent molar mass appears lower.

NCERT 12 Chemistry diagram showing dissociation of KCl into K+ and Cl− in water increasing particle number and lowering apparent molar mass.

Abnormal Molar Mass

As seen above, when solute molecules associate (combine) or dissociate (split) in solution, the number of particles changes.

van’t Hoff Factor (i)

To correct for these changes, van’t Hoff introduced a factor i, defined as:

NCERT 12 Chemistry definition of van’t Hoff factor i as ratios linking normal and abnormal molar masses and observed versus calculated colligative properties.

Interpretation:

Examples: Typical i values

Modified Colligative Property Formulas Using i

Relative lowering of vapour pressure

NCERT 12 Chemistry corrected expression for relative lowering of vapour pressure including the van’t Hoff factor i.

Elevation of boiling point

NCERT 12 Chemistry ebullioscopic elevation of boiling point formula adjusted by van’t Hoff factor i.

Depression of freezing point

NCERT 12 Chemistry cryoscopic depression of freezing point expression corrected using van’t Hoff factor i.

Osmotic pressure

NCERT 12 Chemistry osmotic pressure equation incorporating van’t Hoff factor i for abnormal molar masses.

Key Point

The van’t Hoff factor allows chemists to account for deviations in colligative property-based molar mass calculations caused by molecular association or dissociation in solution. This correction is crucial when dealing with electrolytes and compounds forming aggregates in non-aqueous solvents.

Summary