Last Updated: May 2026
Solutions and Colligative Properties is one of the highest-weightage chapters in CUET 2027 Chemistry — typically 4 to 6 questions every year. The chapter spans NCERT Class 12 Chapter 1 (in the rationalised syllabus) and tests both numerical and conceptual understanding of binary mixtures, vapour pressure, and the four colligative properties.
Quick Reference Table — Colligative Properties
| Property | Formula | What It Depends On |
|---|---|---|
| Relative lowering of vapour pressure | (P°−P)/P° = x_solute | Mole fraction of solute |
| Elevation in boiling point (ΔTb) | Kb × m | Molal concentration × ebullioscopic constant |
| Depression in freezing point (ΔTf) | Kf × m | Molal concentration × cryoscopic constant |
| Osmotic pressure (π) | π = CRT | Molar concentration × R × T |
Concentration Units to Remember
- Molarity (M) — moles solute / litre solution. Temperature-dependent.
- Molality (m) — moles solute / kg solvent. Temperature-independent — preferred for colligative calculations.
- Mole fraction (x) — moles A / total moles. Sum of all mole fractions = 1.
- Mass percentage (w/w) — common in commercial solutions.
- ppm — parts per million; for very dilute solutions like fluoride in water.
Raoult’s Law and Henry’s Law
Raoult’s Law for liquid-liquid solutions: P_total = x_A × P°_A + x_B × P°_B. Henry’s Law for gas-liquid: p = K_H × x. The two laws merge for ideal solutions when the solute is volatile.
Deviations from Raoult’s Law:
- Positive deviation — A-B interactions weaker than A-A, B-B (e.g., ethanol + cyclohexane). Forms minimum boiling azeotrope.
- Negative deviation — A-B stronger (e.g., HCl + water). Forms maximum boiling azeotrope.
van’t Hoff Factor (i)
For electrolytes that dissociate or molecules that associate, observed colligative property differs from expected. The van’t Hoff factor i = observed / calculated, where:
- i > 1 for dissociation (NaCl in water, i ≈ 2)
- i < 1 for association (benzoic acid in benzene, i ≈ 0.5 due to dimerisation)
- i = 1 for non-electrolytes (urea, glucose)
All colligative formulas multiply by i for non-ideal solutes: ΔTb = i × Kb × m, π = iCRT.
High-Yield Numerical Templates
- Find molar mass of unknown solute given depression in freezing point
- Calculate boiling point of NaCl/CaCl₂ solution given Kb of water (0.52 K kg/mol)
- Compute osmotic pressure of glucose solution at body temperature (310 K)
- Determine van’t Hoff factor from observed depression and theoretical depression
- Calculate vapour pressure of solution given mole fraction and pure solvent’s P°
30 Practice MCQs — CUET Chemistry Solutions
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Frequently Asked Questions
Why is molality preferred over molarity in colligative calculations?
Molality uses mass of solvent (which does not change with temperature) while molarity uses volume of solution (which expands or contracts). Colligative properties measured over temperature ranges therefore use molality for accuracy.
Which colligative property is most sensitive at low concentration?
Osmotic pressure. A 1 mM solution exerts ~24 mm Hg at 25°C, easily measurable. Boiling point elevation of the same solution would be only 0.0005 K, essentially undetectable.
What is the van’t Hoff factor for K₄[Fe(CN)₆]?
For complete dissociation, i = 5 (4 K⁺ + 1 [Fe(CN)₆]⁴⁻). In real solutions ion-pairing reduces i below 5.
What is an azeotrope?
A liquid mixture that boils at a constant temperature and distils unchanged in composition. Minimum-boiling azeotropes form from positive deviation; maximum-boiling from negative deviation.
Continue Your CUET 2027 Prep
- CUET Chemistry 2027 — Complete Chapter-wise Syllabus
- CUET Gurukul Courses
- CUET NTA CBT Simulator
- CUET 2027 FAQ
Bottom line: Memorise the four colligative formulas, the four concentration units, when to apply van’t Hoff factor, and the difference between positive/negative deviations from Raoult’s Law.
