Raoult's Law

• Raoult's law for volatile solute: p_A = χ_A p_A° (vapour pressure lowering).
• Ideal vs non-ideal solutions — positive and negative deviations.
• Henry's law for gas in liquid: p = K_H χ — compare with Raoult at low concentration.
• Vapour pressure diagrams and azeotropes (minimum/maximum boiling).
• Colligative property connection — relative lowering of vapour pressure Δp/p° = χ_solute.

Level 1 — Foundations

Verbal: Raoult's law states that the partial vapour pressure of a component in an ideal solution equals its mole fraction times the pure component vapour pressure at the same temperature.

For component A: p_A = χ_A p_A°.

Total pressure (binary volatile): p_total = χ_A p_A° + χ_B p_B°.

Relative lowering: (p_A° − p_A)/p_A° = χ_B (non-volatile solute) — basis for colligative properties.

Ideal solution: Obey Raoult over entire range; ΔH_mix = 0, ΔV_mix = 0 (approximate for similar liquids).

Level 2 — JEE / NEET depth

Positive deviation: p_total > Raoult prediction — weaker A–B interactions (e.g., ethanol–water) — may show minimum boiling azeotrope.

Negative deviation: p_total < Raoult — stronger A–B (e.g., HCl–water) — maximum boiling azeotrope.

Henry's law (dilute gas in solvent): p_gas = K_H χ_gas. K_H ≠ p° when gas differs from solvent.

Phase diagram skills: Plot p vs χ; identify dew point/bubble point qualitatively.

NEET focus: Vapour pressure lowering numerical; identify deviation type from molecular interaction description.

Relation to boiling point elevation: Lower vapour pressure → higher boiling point — connects to next colligative topic.

Vapour pressure lowering by non-volatile solute

Water at 25°C, p° = 24 mm Hg. Add glucose; χ_solute = 0.1 (non-volatile).

Step 1 — χ_water = 1 − 0.1 = 0.9.
Step 2 — p_water = χ_water p° = 0.9 × 24 = 21.6 mm Hg.
Step 3 — Relative lowering = 0.1 = χ_solute ✓.
Step 4 — Boiling point will be slightly elevated.

Ideal binary volatile mixture pressure

Benzene p° = 120 torr, toluene p° = 80 torr; χ_B = 0.6, χ_T = 0.4 (both volatile).

Step 1 — p_B = 0.6 × 120 = 72 torr; p_T = 0.4 × 80 = 32 torr.
Step 2 — p_total = 72 + 32 = 104 torr.
Step 3 — Vapour richer in benzene (more volatile, higher p°).
Step 4 — Composition of vapour uses partial pressures / p_total for mole fractions in gas phase.

• State Raoult's law.
• χ_solute = 0.05 non-volatile — relative lowering?
• Give example of negative deviation.
• Difference between ideal and non-ideal solution?
• Stretch: Explain minimum boiling azeotrope using p–χ diagram.

NCERT Chapter 2 link: Raoult's law connects vapour pressure to mole fraction — foundation for colligative properties. Ideal solution obeys Raoult at all compositions; real solutions show deviations.

Exam connections: Relative lowering Δp/p° = χ₂ for non-volatile solute — links to elevation in boiling point. Positive vs negative deviation molecular explanation — hydrogen bonding vs weak A-B interactions. Azeotrope composition cannot be altered by simple distillation — conceptual point.

Study strategy: Draw vapour pressure-composition diagram for ideal binary solution. For volatile both components, compute partial pressures separately then sum. Henry's law applies to gas solute at low concentration — do not mix with solvent Raoult term incorrectly.

When studying Raoult's Law within Solutions, start by listing every formula and definition on one page without looking at the textbook. Compare your list to NCERT — missing items indicate gaps to fix immediately. Work through at least two NCERT Examples for this section with steps written in full; examiners award method marks even when arithmetic slips.

For board exams (CBSE), long answers benefit from a clear structure: definition → explanation → diagram or formula → example → brief conclusion. Underline key terms. For JEE Main and NEET, prioritise conceptual traps and quick calculation paths; timed mixed quizzes of 10 questions after revision simulate exam pressure.

Cross-topic link: Stoichiometry from Class 11 mole concept underpins solution and electrochemistry numericals.

Spaced revision: Review this note at 1 day, 3 days, and 7 days after first study. Attempt the Quick check questions closed-book, then open the Practice tab for graded reinforcement. Maintain an error log — repeated mistake patterns reveal whether the issue is concept, formula recall, or careless reading.

Diagram and terminology drill: For Chemistry, redraw key figures from memory and define every labelled part in one sentence. Vocabulary precision prevents mark loss in descriptive answers — use NCERT terms exactly as printed in the textbook.

Revision tip: Link this topic to adjacent Class 12 chapters before attempting mixed practice.

Open the Practice tab for graded questions on Raoult's Law.