ChemistryCHEM 04
Solutions & Colligative Properties
Raoult's law, boiling point elevation, freezing point depression, osmotic pressure, van't Hoff factor
Concept Core
Essential theory — everything NCERT tests on Solutions & Colligative Properties
TYPES OF SOLUTIONS & CONCENTRATION
Mole fraction, molality, molarity, normality. Molality (m) preferred for colligative properties (temperature-independent).
Henry's Law: p = KH × x (gas solubility ∝ pressure). Dissolved O₂ in blood, carbonation.
RAOULT'S LAW
Psolution = xsolvent × P°solvent. Relative lowering of VP: (P°−P)/P° = xsolute.
Ideal solution: follows Raoult's law. ΔHmix = 0. ΔVmix = 0. Positive deviation: P > Raoult (alcohol+water). Negative deviation: P < Raoult (chloroform+acetone). Azeotropes form at max/min boiling.
COLLIGATIVE PROPERTIES
Depend only on number of solute particles, NOT their nature.
ΔTb = i × Kb × m (boiling point elevation). ΔTf = i × Kf × m (freezing point depression). π = i × CRT (osmotic pressure).
Kb (water) = 0.52 K·kg/mol. Kf (water) = 1.86 K·kg/mol. Kf > Kb for water.
VAN'T HOFF FACTOR
i = observed/expected particles. Electrolytes dissociate → i > 1. Non-electrolytes: i = 1. Associated solutes: i < 1.
NaCl: i = 2 (ideal). MgCl₂: i = 3. Acetic acid in benzene (dimerises): i = 0.5.
OSMOSIS & ABNORMAL MOLAR MASS
Osmosis: solvent moves through semi-permeable membrane from lower to higher solute conc. π = MRT (dilute solutions). Isotonic: same π. Hypertonic: higher π. Hypotonic: lower π.
Reverse osmosis: applied pressure > osmotic pressure → used in desalination. Abnormal molar mass: calculated from colligative property ≠ actual if association/dissociation occurs.
Fact & Formula Vault
High-yield facts, numbers, and formulas
Key Formulae
ΔTb = i·Kb·m
ΔTf = i·Kf·m
π = i·CRT = i·MRT
Kf(water) = 1.86, Kb(water) = 0.52
van't Hoff Factor
i = 1: non-electrolyte
i > 1: dissociation (NaCl i=2)
i < 1: association (acetic acid in C₆H₆)
MgCl₂: i = 3 (ideal)
Raoult's Law
P = xsolvent × P°
Ideal: ΔHmix=0, ΔVmix=0
Positive deviation: alcohol+water
Negative deviation: CHCl₃+acetone
Worked Examples
NEET-style questions solved step-by-step
EASYWhich colligative property is used to determine molar mass of polymers?▾
Which colligative property is used to determine molar mass of polymers?
Osmotic pressure (π = MRT). Even small concentrations of high molar mass compounds give measurable osmotic pressure. Other colligative properties give too small values for polymers.
MEDIUM0.01 m NaCl solution. ΔTf = ? (Kf = 1.86, i = 2)▾
0.01 m NaCl solution. ΔTf = ? (Kf = 1.86, i = 2)
ΔTf = i × Kf × m = 2 × 1.86 × 0.01 = 0.0372 K. Freezing point = 0 − 0.0372 = −0.0372°C.
HARDAcetic acid dimerises in benzene. If 2 mol/kg solution has i = 0.6. Degree of association?▾
Acetic acid dimerises in benzene. If 2 mol/kg solution has i = 0.6. Degree of association?
For dimerisation: 2CH₃COOH ⇌ (CH₃COOH)₂. i = 1 − α/2. 0.6 = 1 − α/2 → α/2 = 0.4 → α = 0.8 (80% associated).
Mistake DNA
Common NEET traps for this chapter
⚠ Kf vs Kb for water
Kf(water) = 1.86 > Kb(water) = 0.52. A given solute causes more freezing point depression than boiling point elevation for water.
✓ Fix: Kf > Kb for water. Remember: 1.86 and 0.52.
⚠ i for dissociation vs association
Dissociation: i > 1. Association (dimerisation): i < 1. Many students confuse the direction.
✓ Fix: NaCl dissociates → i=2. Acetic acid in benzene associates → i<1.
⚠ Raoult's law deviations
Positive deviation: A-B interactions weaker than A-A/B-B → vapour pressure higher → lower boiling. Negative deviation: A-B stronger → lower VP → higher boiling.
✓ Fix: Positive deviation = forms minimum boiling azeotrope.
Chapter Intelligence
Exam data and last-minute strategy
NEET Frequency
2–3 Q/year. ΔTf and ΔTb calculations with i factor, osmotic pressure for molar mass, Raoult's law deviations, i values for key electrolytes.
High-Yield
Kf=1.86, Kb=0.52 for water. i=2 (NaCl), i=3 (MgCl₂). Osmotic pressure best for polymers. Positive deviation: alcohol+water. Negative: CHCl₃+acetone.
Strategy
Practice ΔTf = i×Kf×m problems. Know Kf and Kb by heart. Identify i for given solutes. These are calculation-based — straightforward with formula.
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