Solutions Class 12 Chemistry Notes: Complete Guide for JEE, NEET & Board Exams

eSaral › Solutions Class 12 Chemistry Notes: Complete Guide for JEE, NEET & Board Exams

Why Solutions Is a High-Priority Chapter in Class 12 Chemistry 

Here is the exam relevance of the Solutions chapter across different tests:

💡 Expert Tip by eSaral Academic Team, IIT Bombay Faculty: "In JEE Main, Solutions questions are almost always numerical — Kb, Kf calculations or van't Hoff factor with electrolytes. In CBSE boards, 3-mark questions often ask you to explain a colligative property with formula and one example. Prepare for both formats separately."

Types of Solutions and Concentration Terms

What Is a Solution?

A solution is a homogeneous mixture of two or more components. The component present in the largest amount is the solvent; the component dissolved in it is the solute.

Solutions can be formed between any combination of solid, liquid, and gas phases. The nine possible types are summarised below:

Ways of Expressing Concentration

Key distinction for exams: Molarity changes with temperature (volume changes). Molality does not change with temperature (mass is constant). For colligative property calculations, always use molality.

India's Best Exam Preparation for Class 12th - Download Now

Solutions | Chemistry Class 12 Notes | IIT-JEE & NEET

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now 

Vapour Pressure and Raoult's Law

Vapour Pressure of a Pure Liquid

The vapour pressure of a liquid is the pressure exerted by its vapour in equilibrium with the liquid at a given temperature. It increases with temperature.

Raoult's Law

For a solution containing a non-volatile solute, Raoult's Law states:

The vapour pressure of a solvent above a solution equals the vapour pressure of the pure solvent multiplied by the mole fraction of the solvent.

p₁ = x₁ × p₁°

Where:

• p₁ = vapour pressure of solvent above the solution
• x₁ = mole fraction of solvent
• p₁° = vapour pressure of pure solvent

Relative lowering of vapour pressure:

(p₁° − p₁) / p₁° = x₂ (mole fraction of solute)

This is the first colligative property.

Ideal vs Non-Ideal Solutions

Colligative Properties: All Four Explained {#colligative-properties}

Colligative properties depend only on the number of solute particles — not their chemical nature. All four properties arise from the lowering of the vapour pressure.

1. Relative Lowering of Vapour Pressure

(p₁° − p₁) / p₁° = x₂ = n₂ / (n₁ + n₂)

For dilute solutions: (p₁° − p₁) / p₁° ≈ n₂/n₁ = w₂M₁ / (M₂w₁)

This is used to determine the molar mass of the solute.

2. Elevation of Boiling Point (ΔTb)

A solution boils at a higher temperature than the pure solvent because its vapour pressure is lower. It reaches atmospheric pressure only at a higher temperature.

ΔTb = Kb × m

Where:

• Kb = ebullioscopic constant (molal elevation constant) — depends only on the solvent
• m = molality of solution

From this: M₂ = (Kb × w₂ × 1000) / (ΔTb × w₁)

Common Kb values: Water = 0.52 K kg/mol | Benzene = 2.53 K kg/mol

3. Depression of Freezing Point (ΔTf)

A solution freezes at a lower temperature than the pure solvent. The solute particles interfere with the formation of the solid lattice.

ΔTf = Kf × m

Where:

• Kf = cryoscopic constant (molal depression constant)
• m = molality

From this: M₂ = (Kf × w₂ × 1000) / (ΔTf × w₁)

Common Kf values: Water = 1.86 K kg/mol | Benzene = 5.12 K kg/mol

Practical application: Anti-freeze in car radiators, salt on icy roads — both work by depressing freezing point.

4. Osmotic Pressure (π)

Osmosis is the spontaneous flow of solvent through a semipermeable membrane from a region of lower solute concentration to higher solute concentration.

π = CRT = (n₂/V)RT

Where:

• π = osmotic pressure
• C = molar concentration of solute
• R = 0.0821 L atm mol⁻¹ K⁻¹
• T = temperature in Kelvin

From osmotic pressure: M₂ = w₂RT / πV

Key terms:

• Isotonic solutions: Equal osmotic pressure — no net flow across the membrane
• Hypertonic solution: Higher osmotic pressure than the reference
• Hypotonic solution: Lower osmotic pressure than the reference

💡 Expert Tip by eSaral Academic Team, IIT Bombay Faculty: "Osmotic pressure is the most sensitive colligative property — it can detect extremely small concentrations. This makes it most useful for determining molar masses of high-molecular-weight substances like proteins and polymers. JEE numericals on osmotic pressure frequently test this application."

Abnormal Molecular Mass and van't Hoff Factor {#vant-hoff}

Why Molecular Mass Appears "Abnormal"

Colligative property calculations assume the solute does not associate or dissociate in solution. When a solute dissociates (like NaCl) or associates (like acetic acid in benzene), the actual number of particles differs from what we calculate — making the measured molar mass appear "abnormal."

van't Hoff Factor (i)

i = observed colligative property / calculated colligative property

Or equivalently:

i = total moles of particles after dissolution / moles of solute dissolved

Modified Colligative Property Formulas with van't Hoff Factor

• ΔTb = i × Kb × m
• ΔTf = i × Kf × m
• π = i × CRT
• Relative lowering = i × x₂

Degree of Dissociation (α)

For an electrolyte AB that dissociates into n ions:

i = 1 + α(n − 1)

Therefore: α = (i − 1) / (n − 1)

Key Formulas 

For complete chapter-wise notes including worked examples, solved numericals, and quick revision sheets for all Class 12 Chemistry chapters, access Complete Class 12 Chemistry Notes on eSaral.

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