JEE Main Chemistry Solutions 2027: Raoult's Law, Colligative Properties & van't Hoff Factor
Raoult's law, ideal and non-ideal solutions, colligative properties and the van't Hoff factor, taught with worked examples and clean formulas.
Edurack Team
September 28, 2026
Why does salt melt ice on a winter road, and why does sea water boil at a slightly higher temperature? Colligative properties answer both. Solutions is a formula-friendly chapter where a handful of relations, applied carefully, decide most JEE Main questions.
Colligative properties depend on how many particles are dissolved, not on what they are.
Chapter at a Glance
| Snapshot | Detail |
|---|---|
| NTA unit | Unit 5 of 20: Solutions |
| Priority (trend-based) | High |
| Typical question style | Colligative property numericals and deviation from Raoult's law concepts |
| Best first step | Learn the four colligative formulas and when i is needed |
Priority reflects past-paper trends, not an official NTA weightage.
What the NTA Syllabus Covers
- Concentration terms: molality, molarity, mole fraction, percentage by volume and mass
- Vapour pressure of solutions, Raoult's law, ideal and non-ideal solutions, vapour pressure-composition plots
- Colligative properties: relative lowering of vapour pressure, depression of freezing point, elevation of boiling point, osmotic pressure
- Molecular mass from colligative properties, abnormal molar mass, van't Hoff factor and its significance
Master These Topics
1. Raoult's Law and Deviations
For a volatile component in an ideal solution, p_A = x_A p_A°. Total pressure of a binary ideal solution is the sum of the partial pressures.
Worked example: With x_A = 0.4, p_A° = 100 mmHg and p_B° = 60 mmHg: P = 0.4 × 100 + 0.6 × 60 = 40 + 36 = 76 mmHg.
Real solutions deviate:
| Type | A-B interactions | ΔH mixing | ΔV mixing | Example |
|---|---|---|---|---|
| Positive deviation | Weaker than A-A and B-B | Positive | Positive | Ethanol and water |
| Negative deviation | Stronger than A-A and B-B | Negative | Negative | Acetone and chloroform |
Henry's law for a gas dissolved in liquid: p = K_H x, with a higher K_H meaning lower solubility.
2. The Four Colligative Properties
- Relative lowering of vapour pressure:
(p° − p) / p° = x_solute - Elevation of boiling point:
ΔT_b = i K_b m - Depression of freezing point:
ΔT_f = i K_f m - Osmotic pressure:
π = i C R T
Worked example (freezing point): 6 g of urea (M = 60) in 500 g of water is 0.1 mol / 0.5 kg = 0.2 molal. With K_f = 1.86 K·kg/mol, ΔT_f = 1.86 × 0.2 = 0.372 K, so the solution freezes at −0.372 °C.
Worked example (osmotic pressure): For a 0.1 M glucose solution at 300 K, π = 0.1 × 0.0821 × 300 ≈ 2.46 atm.
Trap: Use molality for boiling and freezing point formulas and molarity for osmotic pressure. And remember i for electrolytes.
3. The van't Hoff Factor
The factor i corrects for dissociation or association:
- Dissociation into n ions with degree α:
i = 1 + (n − 1)α - Association of n molecules:
i = 1 − α (1 − 1/n)
For a fully dissociated 1:1 electrolyte like NaCl, i ≈ 2, so a 0.1 m solution has ΔT_b = 2 × 0.52 × 0.1 = 0.104 K for water. A value of i below the ideal integer indicates ion pairing, and i below 1 indicates association, as in benzoic acid dimerising in benzene.
4. Molar Mass from Colligative Data
Measure a colligative property, find the moles from the formula, and then M = mass / moles. An abnormally high molar mass indicates association, and a low one indicates dissociation.
Common Traps to Avoid
- Using molarity in the boiling point or freezing point formulas.
- Forgetting the van't Hoff factor for ionic solutes.
- Mixing up positive and negative deviation and the sign of ΔH mixing.
- Confusing a solution's vapour pressure (which drops) with the solvent's pure vapour pressure.
60-Second Revision Sheet
p_A = x_A p_A°;p = K_H xΔT_b = i K_b m,ΔT_f = i K_f m,π = i C R T- Dissociation
i = 1 + (n − 1)α - Positive deviation: ΔH > 0; negative deviation: ΔH < 0
Your Study Plan
- Day 1: concentration terms and conversions between them.
- Day 2: Raoult's law, ideal versus non-ideal solutions and graphs.
- Day 3: all four colligative properties with i.
- Day 4: molar mass problems, abnormal values and a timed mixed set.
Practice Solutions Questions Free → (opens in a new tab)
Continue Your Chemistry Journey
- Previous chapter: Chemical Thermodynamics
- Next chapter: Equilibrium
- All JEE Main Chemistry chapters
- Complete JEE Main Syllabus 2027 guide
Frequently Asked Questions
What is a colligative property?
A property that depends only on the number of solute particles in the solution, not on their identity. The four are relative lowering of vapour pressure, boiling point elevation, freezing point depression and osmotic pressure.
Why do we need the van't Hoff factor?
Because electrolytes split into ions and increase the particle count, while associating solutes reduce it. The factor i corrects the formulas for this.