CBSE Class 12 Solutions PYQs Analysis (2021-2025)

Based on detailed analysis of last 5 years' papers. Perfect for 2026 Boards prep!

Key Trends

Weightage Breakdown: 1-2 MCQs/assertion-reason (1 mark), 1-2 numericals/short answers (2-3 marks), occasional case-based (4 marks) with graphs.
Repeated Topics: Colligative properties, Raoult's law, Henry's law, concentration units.
Difficulty: Easy; numerics on ΔTb/ΔTf/π.
Changes: More numerics on real-life in later years.

Most Important and Repeated Questions

Question Example	Type/Marks	Years Repeated	Notes
Calculate the freezing point of a solution containing 60 g glucose (MM 180) in 250 g water (Kf = 1.86 K kg/mol).	Numerical (2-3 marks)	2022, 2023, 2024, 2025	Repeated 4x; ΔTf ≈ 2.48 K, FP = -2.48°C.
State Raoult's law for non-volatile solute. Calculate vapour pressure of solution with 30 g urea (MM 60) in 846 g water at 298 K (P° = 23.8 mm Hg).	Short Answer/Numerical (3 marks)	2021 Term 2, 2023, 2025	Repeated 3x; P ≈ 23.56 mm Hg.
Assertion: Molality is preferred over molarity for colligative properties. Reason: Molality independent of temperature.	Assertion-Reason (1 mark)	2023, 2024, 2025	Repeated 3x; Both true.
What is Henry's law? Why do scuba divers use helium-diluted air? Calculate solubility of CO2 in water at 298 K under 760 mm Hg (KH = 1.25 × 10^6 mm Hg).	Reasoning/Numerical (2 marks)	2021 Term 1, 2022, 2024	Repeated 3x; x ≈ 0.0008.
Differentiate ideal and non-ideal solutions. Give example of positive deviation (e.g., ethanol + acetone).	Short Answer (2 marks)	2022, 2023, 2025	Repeated 3x; Positive: higher VP.
Calculate molar mass of non-volatile solute: 10 g in 200 g water lowers VP from 32 to 31.84 mm Hg at 308 K.	Numerical (3 marks)	2023, 2024	Repeated 2x; MM ≈ 62 g/mol.
What type of azeotrope is formed by positive deviation from Raoult's law? Give example (e.g., ethanol + water).	Short Answer (2 marks)	2022, 2025	Repeated 2x; Minimum boiling.
Calculate osmotic pressure of 0.2 M glucose at 300 K (R=0.083 L bar/mol K). Is it isotonic with 0.1 M NaCl?	Numerical (2 marks)	2021 Term 2, 2023	Repeated 2x; π ≈ 4.98 bar; yes (NaCl i=2).
MCQ: Which pair shows negative deviation? (a) Chloroform + acetone (b) Phenol + aniline (c) n-Hexane + n-Heptane.	MCQ (1 mark)	2024, 2025	Repeated 2x; (b).
Case-based: Explain colligative properties. Calculate BP elevation for 1 M KCl (i=2, Kb=0.52) vs 1 M sugar (i=1).	Case-Based (4 marks)	2023, 2025	Repeated 2x; ΔTb KCl = 1.04 K.

Best Format for Preparation and Answering

Study Format: Tables for formulas. Practice 10 numerics from PYQs.
Answer Format: Numerics: Formula → Substitute → Calculate + unit. Reasoning: Definition + example.
Tips: Memorize Kf/Kb. Time: 10 mins.

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