Colligative Properties
What are colligative properties?
Colligative properties depend only on the number of dissolved particles, not their identity.
Freezing point:
ΔTf = Kf × m
ΔTf = Kf × m
Boiling point:
ΔTb = Kb × m
ΔTb = Kb × m
- Freezing point depression
- Boiling point elevation
- Osmotic pressure
Formulas
Freezing point depression:
ΔTf = Kf × m
ΔTf = Kf × m
Boiling point elevation:
ΔTb = Kb × m
ΔTb = Kb × m
Molality:
m = ΔT / K
m = ΔT / K
For electrolytes (extended):
ΔT = i × K × m
ΔT = i × K × m
Examples
Acetic acid + solute
Kf=3.90; m=0.5
ΔTf=1.950°C
Kf=3.90; m=0.5
ΔTf=1.950°C
Boiling point (acetic acid)
Kb=3.07; m=0.5
ΔTb=1.535°C
Kb=3.07; m=0.5
ΔTb=1.535°C
Polar protic solvent
Acetic acid shows pronounced freezing and boiling shifts.
Acetic acid shows pronounced freezing and boiling shifts.
Process insight
Larger K values produce larger ΔT at the same molality.
Larger K values produce larger ΔT at the same molality.
Technical Background
Importance of molality
Molality is temperature-independent and therefore ideal for colligative calculations.
Applications
- Cryoscopy and ebullioscopy
- Antifreeze formulation
- Molar mass determination
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