Chemistry: Solutions - Lecture 5 | CHEMCA

Class 12 / JEE / NEET Chapter 1

Chemistry - Solutions | Lecture 5

By Abhishek Sengar

Lecture Chapters

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Theory & Quick Notes

1 Intro to Colligative Properties

Colligative properties are properties of solutions that depend entirely on the ratio of the number of solute particles to the number of solvent particles in a solution, and not on the nature or identity of the chemical species present.

The 4 Main Colligative Properties:

Relative Lowering of Vapor Pressure (RLVP)
Elevation in Boiling Point (ΔT_b)
Depression in Freezing Point (ΔT_f)
Osmotic Pressure (π)

2 Relative Lowering of Vapor Pressure (RLVP)

When a non-volatile solute is added to a volatile solvent, the vapor pressure of the solvent decreases. The relative lowering of vapor pressure is directly equal to the mole fraction of the solute.

Exact Formula

(P°_A - P_S) / P°_A = x_B

Where x_B is the mole fraction of the solute: n_B / (n_A + n_B)

DILUTE RULE

For Dilute Solutions

If n_A >> n_B, then (n_A + n_B) ≈ n_A

(P°_A - P_S) / P°_A ≈ n_B / n_A

3 Boiling Point & Freezing Point Changes

Elevation in Boiling Point (ΔT_b)

Adding a non-volatile solute lowers the vapor pressure. Thus, a higher temperature is required for the vapor pressure to equal 1 atm (external pressure). Result: Boiling point increases.

ΔT_b = T_b - T°_b = K_b × m

K_b = Ebullioscopic Constant. m = molality.

Depression in Freezing Point (ΔT_f)

The freezing point is the temperature where the vapor pressure of the liquid phase equals that of the solid phase. Because the solute lowers vapor pressure, this intersection happens at a lower temperature.

ΔT_f = T°_f - T_f = K_f × m

K_f = Cryoscopic Constant. m = molality.

4 Thermodynamic Expressions for K_b and K_f

Crucial for JEE/NEET Advanced problems. The constants depend only on the properties of the pure solvent.

K_b = (R × M₁ × T°_b²) / (1000 × ΔH_vap)

K_f = (R × M₁ × T°_f²) / (1000 × ΔH_fus)

R = 8.314 J/(mol·K), M₁ = Molar mass of solvent (g/mol), Temperatures must be in Kelvin.

Deep Dive

Want the Complete Detailed Notes?

Read the full comprehensive article for Chapter 1 on our blog, including detailed theory, solved examples, and deep dives for JEE/NEET.

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Test Your Knowledge

Lecture 5 Checkpoint

Answer these quick questions to solidify your understanding of Colligative Properties.

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