Colligative Properties
World B — Acids, Bases & Solutions
20.A — VAPOR PRESSURE & BOILING
=
R20.01 The Core Concept
- Properties depending ONLY on the number of solute particles.
- Nature/identity of the particle does not matter.
- Must be a non-volatile solute in a volatile solvent.
R20.02 Vapor Pressure Lowering
- Solute particles occupy surface area.
- Fewer solvent molecules can escape into gas phase.
- Vapor pressure of solution < Vapor pressure of pure solvent.
P = P°X
Mole Fraction
R20.03 Raoult's Law
- Partial vapor pressure is proportional to mole fraction.
- $\Delta P = P°_A \cdot X_B$
- Basis for all colligative property math.
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R20.04 Boiling Point Elevation
- Lower VP means more heat is needed to reach atmospheric pressure.
- Solution boils at a higher temp than pure solvent.
- $\Delta T_b = K_b \cdot m$ (molality).
20.B — FREEZING & OSMOSIS
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R20.05 Freezing Point Depression
- Solute particles disrupt crystal lattice formation.
- Must cool to a lower temp to freeze solid.
- $\Delta T_f = K_f \cdot m$.
R20.06 Osmosis
- Net flow of solvent through a semi-permeable membrane.
- Flows from dilute solution → concentrated solution.
- Seeks to equalize concentration.
↓
R20.07 Osmotic Pressure (Π)
- Pressure required to entirely stop osmosis.
- $\Pi = CRT$ (Concentration, Gas Const, Temp).
- Used to find molar mass of large polymers/proteins.
← H₂O
R20.08 Reverse Osmosis (RO)
- Apply external pressure > Osmotic pressure.
- Forces pure solvent out of the concentrated solution.
- Used for seawater desalination.
20.C — VAN 'T HOFF & APPLICATIONS
ISO
HYPER
HYPO
R20.09 Tonicity in Biology
- Isotonic: No net flow (IV fluids).
- Hypertonic: Water leaves cell (Plasmolysis/Shrivel).
- Hypotonic: Water enters cell (Hemolysis/Burst).
→
R20.10 Van 't Hoff Factor (i)
- Accounts for dissociation of electrolytes.
- NaCl → Na⁺ + Cl⁻ (i = 2).
- Multiplies the colligative effect ($\Delta T = i \cdot K \cdot m$).
→
R20.11 Abnormal Molar Mass
- Occurs when solutes associate or dissociate.
- Association (e.g., Acetic acid dimerizes): i < 1.
- Calculated molar mass appears higher/lower than expected.
R20.12 Real-World Applications
- De-icing: Salt on roads lowers freezing point of ice.
- Radiator Coolant: Ethylene glycol prevents freezing in winter, boiling in summer.