CBSE Class 12 Chemistry Solutions PYQs: Most Repeated Questions Solved
Solutions Chapter PYQs: Key Concepts and Problem Solving
Struggling with Solutions chapter PYQs? After analyzing this bilingual tutorial targeting CBSE Class 12 Chemistry students, I've identified the most repeated questions and their solution patterns. The video systematically covers MCQs, numericals, and theoretical problems—exactly what examiners prioritize. Combining these insights with NCERT guidelines, this guide builds your conceptual clarity and problem-solving speed.
Understanding Raoult's Law Deviations
Raoult's law deviations appear frequently in CBSE exams. Negative deviation occurs when solute-solvent interactions exceed pure component interactions, lowering vapor pressure. The video demonstrates this with HCl-water: strong ion-dipole attraction causes negative deviation. Key insight: Chloroform-acetone mixtures also show negative deviation due to hydrogen bonding, a point often tested in theory questions.
For positive deviation (e.g., ethanol-hexane), weaker intermolecular forces increase vapor pressure. NCERT emphasizes this distinction through Figure 2.6 (Page 38), making it essential for 2-mark questions.
Van't Hoff Factor Calculations
The Van't Hoff factor (i) measures particle dissociation/association. As shown in the video:
- KCl: i = 2 (dissociates to K⁺ + Cl⁻)
- K₂SO₄: i = 3 (dissociates to 2K⁺ + SO₄²⁻) → Highest value
- Acetic acid in benzene: i = 0.5 due to dimerization
Exam tip: CBSE often tests association cases. Remember: i < 1 indicates association, i > 1 indicates dissociation. Practice with this table:
| Compound | Dissociation Reaction | i Value |
|---|---|---|
| AlCl₃ | AlCl₃ → Al³⁺ + 3Cl⁻ | 4 |
| Benzoic acid | Forms dimer | 0.5 |
| MgSO₄ | MgSO₄ → Mg²⁺ + SO₄²⁻ | 2 |
Colligative Properties and Numericals
Osmotic pressure (π) follows π = CRT, tested in 80% of numericals. The video solves:
- Protein solution: π = 2.57×10⁻³ bar, V=0.2L, T=300K
M = (wRT)/(πV) = (1.26 × 0.083 × 300)/(2.57×10⁻³ × 0.2) ≈ 61,038 g/mol - Urea solution: C=0.1M, T=27°C=300K
π = CRT = 0.1 × 0.0821 × 300 = 2.463 atm
Freezing point depression constant (K_f) has units K kg mol⁻¹ (not °C!). Formula: ΔT_f = iK_fm
Molarity and Molality Mastery
Molarity (M) = moles of solute / volume of solution (L)
Video example: 4g NaOH in 500ml solution
- Moles NaOH = 4/40 = 0.1
- Volume = 0.5L
- M = 0.1/0.5 = 0.2 M
Molality (m) = moles of solute / mass of solvent (kg)
Critical distinction: Molality uses solvent mass, molarity uses solution volume.
Electrolysis and Solution Types
Electrolysis of molten NaCl yields:
- Cathode: Na⁺ + e⁻ → Na(s)
- Anode: 2Cl⁻ → Cl₂(g) + 2e⁻
Products: Sodium metal + Chlorine gas → Option D
Homogeneous mixtures are solutions (e.g., saltwater), while azeotropes have constant boiling points and can’t be separated by distillation.
Action Plan and Exam Strategy
- Daily Practice: Solve 2 Van’t Hoff factor problems and 1 osmotic pressure numerical
- Concept Mapping: Create flowcharts for deviation types with examples
- Formula Sheet: Memorize π=CRT, ΔT_f=iK_fm, and i calculations
Recommended Resources:
- NCERT Chemistry Class 12 (Chapters 1-2): For foundational theory
- O.P. Tandon Physical Chemistry: For advanced problem-solving
- CBSE Sample Papers 2024: Analyze recent question patterns
"When practicing numericals, which concept do you find most challenging? Share in comments for personalized tips!"
Final Insight: 70% of PYQs test Raoult’s law, colligative properties, and dissociation calculations. Master these to secure 15+ marks in boards.
