📘 MP Board Class 12 Chemistry Chapter 1: Solutions — Complete Notes for 2027 Exam

Chapter 1: Solutions is one of the most scoring chapters in your MP Board Class 12 Chemistry syllabus. This chapter lays the foundation for physical chemistry and carries significant weight in board exams (typically 10–14 marks). In these comprehensive notes, we cover every key concept — from concentration terms to colligative properties — with clear formulas and practical examples tailored to the MP Board 2027 exam pattern.

🧪 1. Types of Solutions

A solution is a homogeneous mixture of two or more substances. The component present in larger quantity is called the solvent, and the one present in smaller quantity is called the solute.

🔢 2. Expressing Concentration of Solutions

MP Board exams frequently ask numerical problems on concentration terms. Memorise the following formulas — they are your key to scoring full marks in the numerical section.

💧 3. Solubility — Factors & Henry’s Law

Factors Affecting Solubility

• Nature of solute and solvent — “Like dissolves like” (polar solutes in polar solvents, non-polar in non-polar)
• Temperature — For most solids, solubility increases with temperature (endothermic dissolution). For gases, solubility decreases with increasing temperature.
• Pressure — Affects solubility of gases only. Governed by Henry’s Law.

Henry’s Law

Statement: At constant temperature, the mass of a gas dissolved in a given volume of liquid is directly proportional to the pressure of the gas above the liquid.

Mathematical form: p = K_H × x

Where p = partial pressure of the gas, K_H = Henry’s law constant, x = mole fraction of the gas in solution.

Important: Higher the value of K_H, lower the solubility of the gas at the same partial pressure. For example, K_H for N₂ (6.5×10⁴ bar) > K_H for O₂ (3.3×10⁴ bar), meaning O₂ is more soluble than N₂ in water.

📊 4. Vapour Pressure & Raoult’s Law

Vapour Pressure of Liquid Solutions

When a non-volatile solute is dissolved in a volatile solvent, the vapour pressure of the solution is lower than that of the pure solvent. This is the basis for all colligative properties.

Raoult’s Law

For a solution of volatile liquids: The partial vapour pressure of each component is directly proportional to its mole fraction in the solution.

p_A = p_A° × x_A  and  p_B = p_B° × x_B

Total vapour pressure: p_total = p_A + p_B = p_A°x_A + p_B°x_B

📐 5. Colligative Properties

Colligative properties depend only on the number of solute particles, not their chemical identity. These are the most numerically important sections for the MP Board Chemistry exam.

Key Points for Numerical Problems

• Use the formula: Molar mass (M) = (K_f × w_B × 1000) / (ΔT_f × w_A), where w_B = mass of solute (g), w_A = mass of solvent (g)
• Osmotic pressure method is preferred for determining molar mass of macromolecules (proteins, polymers) because its magnitude is large even at low concentrations
• Q₁₀: The temperature coefficient — most colligative property problems are solved with standard K_b and K_f values given in the question paper

🧮 6. Abnormal Molar Masses — van’t Hoff Factor

When a solute undergoes association (molecules combine) or dissociation (molecules break into ions) in solution, the observed molar mass differs from the theoretical value. This is corrected using the van’t Hoff factor (i).

i = Normal molar mass / Observed molar mass = Observed colligative property / Normal colligative property

For dissociation: i > 1 (e.g., NaCl → Na⁺ + Cl⁻, so i = 2)
For association: i < 1 (e.g., benzoic acid in benzene dimerises, so i = 0.5)

❓ 7. Frequently Asked Questions (FAQs)

Q1: What is the difference between molarity and molality?

Answer: Molarity (M) is moles of solute per litre of solution — it depends on temperature because volume changes with temperature. Molality (m) is moles of solute per kg of solvent — it is temperature-independent because mass is constant. For MP Board numericals, use molality when dealing with colligative properties like boiling point elevation or freezing point depression.

Q2: What are ideal and non-ideal solutions?

Answer: Ideal solutions obey Raoult’s law at all concentrations — the intermolecular forces between A-A, B-B, and A-B are equal (e.g., benzene + toluene, n-hexane + n-heptane). Non-ideal solutions deviate from Raoult’s law — they can show either positive deviation (A-B interactions are weaker than A-A and B-B) or negative deviation (A-B interactions are stronger).

Q3: Why is sea water not suitable for drinking?

Answer: Sea water has a high osmotic pressure due to dissolved salts. When consumed, the osmotic pressure difference causes dehydration of body cells (exosmosis). This is why drinking sea water increases thirst and can lead to kidney damage. Reverse osmosis is used to desalinate sea water for drinking purposes.

Q4: What is the significance of the van’t Hoff factor?

Answer: The van’t Hoff factor (i) corrects the colligative properties for the actual number of particles in solution. When a solute dissociates (like NaCl splitting into Na⁺ and Cl⁻), i > 1, and the observed depression in freezing point is larger than expected. When a solute associates (like benzoic acid forming dimers in benzene), i < 1, and the observed colligative effect is smaller. This concept appears in almost every MP Board numerical section.

Q5: How is freezing point depression used to determine molar mass?

Answer: By measuring ΔT_f experimentally (using a Beckmann thermometer or cryoscope), the molar mass of the solute can be calculated using: M_B = (K_f × w_B × 1000) / (ΔT_f × w_A). This cryoscopic method is widely used for determining molar masses of non-volatile, non-electrolytic substances in the laboratory.

Q6: What is an azeotropic mixture?

Answer: An azeotrope is a mixture of two liquids that boils at a constant temperature and has the same composition in the liquid and vapour phases. The ethanol-water azeotrope (95.4% ethanol, 4.6% water, bp 78.15°C) is the most famous example — this is why 100% ethanol cannot be obtained by simple fractional distillation. Azeotropes can be minimum-boiling (positive deviation) or maximum-boiling (negative deviation).

Q7: What is reverse osmosis and where is it used?

Answer: When pressure greater than the osmotic pressure is applied on the solution side, solvent molecules move from the solution to the pure solvent through a semipermeable membrane — this is reverse osmosis. It is used for desalination of sea water, water purification (RO water filters), and wastewater treatment. MP Board often includes this as a 2-mark application question.

📚 MP Board Class 12 Chemistry Chapter 1 Solutions Notes — Prepared for MP Board 2027 Exam. Visit mpboard.ai for more chapter-wise notes, PYQs, and study material.