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APEX CLASS ESTD 2018

IITJEE CHEMISTRY: Solutions, Raoult’s Law Deviations & Colligative Properties

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Ashish Kumar Bahekar

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APEX CLASS • IITJEE Physical Chemistry Lab

Solutions, Raoult’s Law Deviations & Colligative Properties

Master Vapor Pressure Graphs, Ideal vs Non-Ideal Deviations, BP/FP Phase Curves, and Van ‘t Hoff Factor (i)!

🔬 Ptotal = P°AXA + P°BXB. Select a Graph:
XA = 1 XB = 0 XA = 0 XB = 1 Mole Fraction → Vapor Pressure P°A P°B PA PB Ptotal = PA + PB Ideal Solution Properties 1. Raoult’s Law Strictly Obeyed: PA = P°A XA and PB = P°B XB 2. Intermolecular Forces: A-B interactions ≈ A-A and B-B 3. Zero Thermodynamics Changes: ΔHmix = 0 and ΔVmix = 0 XA = 1 XA = 0 P°A P°B Ptotal > Pideal Positive Deviation Rules • Interactions: A-B < A-A or B-B (Molecules escape easily, VP increases!) • Thermodynamics: ΔHmix > 0, ΔVmix > 0 (Endothermic, volume expands slightly) • Azeotrope: Minimum Boiling Azeotrope Ex: Ethanol + Water, Ethanol + Acetone XA = 1 XA = 0 Ptotal < Pideal Negative Deviation Rules • Interactions: A-B > A-A or B-B (Stronger bonds trap molecules, VP drops!) • Thermodynamics: ΔHmix < 0, ΔVmix < 0 (Exothermic, volume contracts slightly) • Azeotrope: Maximum Boiling Azeotrope Ex: Chloroform + Acetone (H-bonding!) Henry’s Law (Solubility of a Gas in a Liquid) P = KH × Xgas in liquid P Mole Fraction X → Slope = KH 1. Higher KH = LOWER Gas Solubility! (Since X = P / KH, they are inversely related). 2. KH increases with Temperature! (Warm water holds LESS oxygen ⇒ bad for fish).
Mole Fraction (XA)
nA / (nA + nB)
Sum of all X = 1. Temp Independent!
Vapor Phase Comp (YA)
YA = PA / Ptotal
Dalton’s Law inside the vapor.
Azeotropic Mixture
Boils at Constant Temp
Liquid & Vapor comps are identical.
Henry’s Constant KH
Depends on Gas Nature
He > N₂ > O₂ (He is least soluble).
🧪 Depend ONLY on number of solute particles. Select a Colligative Property:
Relative Lowering of Vapor Pressure (RLVP) Pure Solvent (P°) → Solution (Ps) Non-volatile Solute ( P° – Ps ) / P° = i × Xsolute Solute particles block the surface, reducing the rate of evaporation! Note: (P° – P) is Lowering of VP. Division by P° makes it RLVP. V.P. Temp (T) → 1 atm Solid Pure Solvent Solution (V.P. Dropped!) T°b Tb T°f Tf ΔTb ΔTf Ebullioscopy & Cryoscopy 1. BP Elevation: ΔTb = i · Kb · m Solution boils at a HIGHER temp because it needs more heat to reach 1 atm VP! 2. FP Depression: ΔTf = i · Kf · m Solution freezes at a LOWER temp because solute disrupts the solid lattice formation. Note: m = Molality (Temperature Independent!) SPM Pure Solvent Solution (Conc C) Solvent Flow Pressure π Osmotic Pressure (π) & Reverse Osmosis π = i · C · R · T • C = Molarity (mol/L). The only Colligative property using Molarity! • Isotonic Solutions: π₁ = π₂ (Same osmotic pressure) If i₁=i₂=1, then C₁ = C₂. • Reverse Osmosis (RO): Apply external pressure P > π. Solvent flows BACKWARDS from Solution to Pure Solvent (Desalination). Van ‘t Hoff Factor (i): Accounting for Dissociation & Association 1. Dissociation (i > 1) i = 1 + (n – 1)α • n = particles formed (e.g. K₂SO₄ → n=3) • α = Degree of Dissociation 2. Association (i < 1) i = 1 + (1/n – 1)α • n = particles associating (e.g. Dimer n=2) • Ex: Acetic Acid dimerizes in Benzene! Observed Colligative Property = i × (Calculated Theoretical Property) 🏆 JEE Trick: If asked for “Highest Boiling Point”, calculate (i × m) for all options. The highest value wins!
Ebullioscopic Const (Kb)
For Water: 0.52 K·kg/mol
Depends ONLY on the solvent!
Cryoscopic Const (Kf)
For Water: 1.86 K·kg/mol
Kf = (R × M × T°f²) / (1000 × ΔHfus)
Van ‘t Hoff & Molar Mass
i = Mtheoretical / Mobserved
Dissociation causes Observed Mass to drop!
Osmotic Pressure (π)
Best for Macromolecules
Measured at Room Temp (Proteins/Polymers)
🔥 Azeotropic Distillation Trap:
A solution with a Positive Deviation from Raoult’s Law has a higher vapor pressure than ideal, meaning it boils easier. It forms a Minimum Boiling Azeotrope (e.g., 95.5% Ethanol + Water). A solution with a Negative Deviation forms a Maximum Boiling Azeotrope (e.g., 68% Nitric Acid + Water).
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