Class 12 Chemistry: Solutions - Revision Notes

Introduction

The chapter "Solutions" in Class 12 Chemistry covers essential concepts like types of solutions, concentration terms, Raoult's Law, colligative properties, and more. Understanding these concepts is crucial for both board exams and competitive exams like JEE and NEET. In this post, we'll dive deep into the various topics and provide detailed solutions with all the necessary formulae.

Types of Solutions

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

Types Based on Solvent and Solute:

• Solid in Liquid Solutions: Example - Sugar in water.
• Gas in Liquid Solutions: Example - Carbon dioxide in water (soda water).
• Liquid in Liquid Solutions: Example - Alcohol in water.

Concentration Terms and Their Formulae

Understanding concentration terms is vital for solving problems in chemistry. Here are the most important concentration terms:

1. Molarity (M): Number of moles of solute per liter of solution.
Molarity (M) = (Moles of solute) / (Volume of solution in liters)

2. Molality (m): Number of moles of solute per kilogram of solvent.
Molality (m) = (Moles of solute) / (Mass of solvent in kg)

3. Normality (N): Number of gram equivalents of solute per liter of solution.
Normality (N) = (Gram equivalents of solute) / (Volume of solution in liters)

4. Mole Fraction (χ): Ratio of the number of moles of a component to the total number of moles in the solution.
Mole Fraction of A (χ_A) = (Number of moles of A) / (Total number of moles of all components)

5. Parts per Million (ppm): Number of parts of solute per million parts of solution.
ppm = (Mass of solute / Mass of solution) × 10⁶

Raoult's Law

Raoult's Law relates the partial vapor pressure of a component in a solution to its mole fraction.

For ideal solutions: The partial vapor pressure of each component is directly proportional to its mole fraction.

P_A = P_A⁰ ⋅ χ_A
P_B = P_B⁰ ⋅ χ_B

Where:

• P_A and P_B are the partial pressures of components A and B.
• P_A⁰ and P_B⁰ are the vapor pressures of pure components A and B.
• χ_A and χ_B are the mole fractions of A and B.

Colligative Properties

Colligative properties depend on the number of solute particles in a solution rather than their identity. There are four primary colligative properties:

1. Relative Lowering of Vapor Pressure:
ΔP = P₁⁰ - P₁ = P₁⁰ ⋅ χ₂

2. Elevation of Boiling Point (ΔT_b):

ΔT_b = K_b · m

3. Depression of Freezing Point (ΔT_f):

ΔT_f = K_f · m

4. Osmotic Pressure (π):

π = MRT

Van’t Hoff Factor (i)

The Van't Hoff factor, denoted as i, is used to account for the effect of ionization or association in colligative properties. It is given by:

i = (Actual number of particles in solution after dissociation) / (Number of particles in solution before dissociation)

Practice Problems

Problem 1:

Calculate the molality of a solution containing 20 grams of NaOH in 500 grams of water.

Problem 2:

A solution is prepared by dissolving 2 moles of a non-volatile solute in 1 kg of water. Calculate the elevation in boiling point if K_b = 0.52 K kg/mol.

Problem 3:

Calculate the osmotic pressure of a solution containing 1 gram of glucose (C₆H₁₂O₆) in 100 mL of water at 25°C (R = 0.0821 L atm/mol K).

Conclusion

Understanding the concepts of solutions in Chemistry is crucial for mastering the subject. Practice these problems, learn the formulae, and understand their applications to ace both your board exams and competitive exams like JEE and NEET. Keep revising and practicing!

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