Class 12 Chemistry Solutions Complete Guide 2027 — Raoult's Law, Colligative Properties, Numericals

In short: Solutions carries 6-7 marks in CBSE Class 12 Chemistry 2027 and is the most numerical-heavy chapter in Physical Chemistry. Four colligative properties — relative lowering of vapour pressure, elevation in boiling point, depression in freezing point, and osmotic pressure — each have a formula. Learn the formulas, understand Raoult's law and its deviations, and practise the numericals. This chapter follows the same pattern as Class 10 Trigonometry: formula-driven with predictable question types.

Solutions is Chapter 2 in CBSE Class 12 Chemistry and one of the highest-scoring Physical Chemistry chapters on the board paper. It carries 6-7 marks, typically split between one numerical (3-5 marks) and one conceptual question (2-3 marks). The chapter connects directly to your practical exam — determining molecular mass using colligative properties is a common practical question.

For the full marks breakdown, see the Class 12 Chemistry chapter-wise weightage. For all formulas in one place, check the Class 12 Chemistry formulas list.

Concentration Terms You Must Know

Memory trick: Molarity uses volume of SOLUTION (changes with temperature). Molality uses mass of SOLVENT (does NOT change with temperature). Board exams love asking: "Which concentration term is temperature-independent?" Answer: molality, mole fraction, mass percentage.

Raoult's Law

For volatile solute in volatile solvent:

P_A = χ_A × P°_A and P_B = χ_B × P°_B

Total vapour pressure: P_total = P_A + P_B = χ_A P°_A + χ_B P°_B

Where P° = vapour pressure of pure component, χ = mole fraction.

For non-volatile solute in volatile solvent:

P = χ_solvent × P°_solvent

Relative lowering of vapour pressure: (P° − P)/P° = χ_solute = n₂/(n₁ + n₂)

Ideal solutions: Follow Raoult's law exactly. ΔH_mix = 0, ΔV_mix = 0. Example: benzene + toluene, hexane + heptane.

Non-ideal solutions with positive deviation: P_total > expected by Raoult's law. Solute-solvent interactions are WEAKER than pure component interactions. Example: ethanol + water, acetone + carbon disulphide. Show positive deviation, form minimum boiling azeotropes.

Non-ideal solutions with negative deviation: P_total < expected. Solute-solvent interactions are STRONGER. Example: chloroform + acetone, HCl + water. Show negative deviation, form maximum boiling azeotropes.

Memory trick: "Positive = Push apart (weaker forces, higher vapour pressure). Negative = Nestle closer (stronger forces, lower vapour pressure)."

The Four Colligative Properties

Colligative properties depend only on the NUMBER of solute particles, not their nature.

1. Relative Lowering of Vapour Pressure

(P° − P)/P° = χ_solute = n₂/(n₁ + n₂) ≈ n₂/n₁ (for dilute solutions)

2. Elevation in Boiling Point

ΔT_b = K_b × m

Where K_b = ebullioscopic constant (given), m = molality.

ΔT_b = K_b × (w₂ × 1000)/(M₂ × w₁)

Where w₂ = mass of solute, M₂ = molar mass of solute, w₁ = mass of solvent in grams.

3. Depression in Freezing Point

ΔT_f = K_f × m

Where K_f = cryoscopic constant (given), m = molality.

ΔT_f = K_f × (w₂ × 1000)/(M₂ × w₁)

4. Osmotic Pressure

π = CRT (where C = molarity, R = gas constant, T = temperature in Kelvin)

Or: π = (n₂/V)RT = (w₂RT)/(M₂V)

Memory trick for all four: "VBFO" — Vapour pressure lowering, Boiling point elevation, Freezing point depression, Osmotic pressure. All depend on the number of solute particles.

Van't Hoff Factor (i)

For electrolytes that dissociate or molecules that associate in solution:

i = observed colligative property / calculated colligative property

i = (actual number of particles in solution) / (formula units dissolved)

For association: i < 1 (fewer particles than expected) For dissociation: i > 1 (more particles than expected)

Modified formulas:

• ΔT_b = i × K_b × m
• ΔT_f = i × K_f × m
• π = i × CRT

Example: NaCl → Na⁺ + Cl⁻ (i = 2 for complete dissociation) K₂SO₄ → 2K⁺ + SO₄²⁻ (i = 3) Acetic acid in benzene (dimerises): i = 0.5

Board Exam Question Types — Solved

Type 1: Calculate Molar Mass from Colligative Property (3-5 marks)

"2.0 g of a non-volatile solute is dissolved in 100 g of water. The boiling point is elevated by 0.52°C. Find the molar mass. (K_b = 0.52 K·kg/mol)"

ΔT_b = K_b × (w₂ × 1000)/(M₂ × w₁) 0.52 = 0.52 × (2 × 1000)/(M₂ × 100) M₂ = (0.52 × 2000)/(0.52 × 100) = 20 g/mol

Type 2: Positive/Negative Deviation Explanation (2-3 marks)

Explain with examples. Draw vapour pressure vs composition graphs showing deviations from Raoult's law. Board loves: "Why does ethanol-water show positive deviation?"

Type 3: Osmotic Pressure Numerical (3 marks)

"Find the osmotic pressure of 0.1 M glucose at 27°C." π = CRT = 0.1 × 0.0821 × 300 = 2.463 atm

Type 4: Van't Hoff Factor (2-3 marks)

Calculate i from degree of dissociation, or use i to modify colligative property calculations.

Common Mistakes That Cost Marks

1. Using mass of solution instead of mass of solvent in molality. Molality = moles of solute / kg of SOLVENT (not solution). This error changes the entire answer.

2. Forgetting to convert temperature to Kelvin for osmotic pressure. π = CRT requires T in Kelvin. 27°C = 300 K. Using 27 instead of 300 gives an answer off by a factor of 11.

3. Not applying van't Hoff factor for electrolytes. If the solute is NaCl, KCl, CaCl₂, etc., you MUST multiply by i. Forgetting i gives the wrong molar mass.

4. Confusing positive and negative deviations. Positive deviation = weaker interactions = higher vapour pressure. Negative = stronger interactions = lower vapour pressure. Draw the graph to check your reasoning.

5. Mixing up K_b and K_f. K_b is for boiling point elevation, K_f is for freezing point depression. They have different values for the same solvent. Read the question carefully to see which one is given.

4-Week Practice Plan

Week 1: Concentration Terms and Raoult's Law

• Read NCERT Sections 2.1-2.4
• Practice converting between molarity, molality, and mole fraction (10 problems)
• Solve Raoult's law problems for both volatile and non-volatile solutes

Week 2: Colligative Properties

• Read NCERT Sections 2.5-2.8
• Solve 5 ΔT_b problems, 5 ΔT_f problems, and 5 osmotic pressure problems
• Practice finding molar mass from each colligative property

Week 3: Van't Hoff Factor and Deviations

• Read NCERT Sections on deviations and van't Hoff factor
• Solve 5 problems involving electrolytes (using i)
• Practice explaining positive and negative deviations with diagrams
• Attempt CBSE Class 12 Chemistry important chapters for this topic

Week 4: Board Paper Practice

• Solve Solutions questions from the last 5 years of CBSE board papers
• Time yourself: 5-mark numericals in 10 minutes
• Focus on "find molar mass" type questions — these are the most common
• Final drill: one numerical + one deviation explanation in 12 minutes

Quick Revision Card for Exam Day

• Molality = mol solute / kg SOLVENT (temperature independent)
• Raoult's law: P = χ × P° for each component
• ΔT_b = i·K_b·m, ΔT_f = i·K_f·m, π = iCRT
• M₂ = (K × w₂ × 1000)/(ΔT × w₁)
• Positive deviation = weaker forces = higher P (ethanol + water)
• Negative deviation = stronger forces = lower P (CHCl₃ + acetone)
• i > 1 for dissociation, i < 1 for association
• NaCl: i=2, CaCl₂: i=3, K₂SO₄: i=3
• Convert °C to K for osmotic pressure (add 273)

For step-by-step numerical practice on colligative properties, try Padhaao's AI tutor.

Frequently Asked Questions

How many marks does Solutions carry in CBSE Class 12 Chemistry 2027?

Solutions carries 6-7 marks in CBSE Class 12 Chemistry 2027. It is one of the highest-weighted Physical Chemistry chapters. Expect one 3-5 mark numerical (finding molar mass using colligative properties) and one 2-3 mark conceptual question (Raoult's law deviations or ideal vs non-ideal solutions).

What are colligative properties and why are they called colligative?

Colligative properties depend only on the NUMBER of solute particles, not on their chemical identity. The four colligative properties are: relative lowering of vapour pressure, elevation in boiling point, depression in freezing point, and osmotic pressure. The word "colligative" comes from Latin "colligatus" meaning "bound together" — these properties are bound to the quantity of solute particles regardless of what they are.

What is the van't Hoff factor and when do I use it?

The van't Hoff factor (i) corrects colligative property calculations for solutes that dissociate (like NaCl → Na⁺ + Cl⁻, i = 2) or associate (like acetic acid dimerising in benzene, i ≈ 0.5) in solution. Multiply i into all colligative property formulas: ΔT_b = iK_bm, ΔT_f = iK_fm, π = iCRT. If the solute is a non-electrolyte (glucose, urea), i = 1 and no correction is needed.

What is the difference between ideal and non-ideal solutions?

Ideal solutions obey Raoult's law exactly — the intermolecular forces between solute and solvent are identical to those in pure components. ΔH_mix = 0, ΔV_mix = 0. Examples: benzene + toluene. Non-ideal solutions deviate from Raoult's law because solute-solvent interactions differ from pure component interactions. Positive deviation means weaker interactions (higher vapour pressure), negative deviation means stronger interactions (lower vapour pressure).

Solutions is a formula-driven chapter where numerical accuracy matters most. Know your four colligative property formulas, remember the van't Hoff factor for electrolytes, and convert temperature to Kelvin — those 6-7 marks reward careful calculation.