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Experiment on Colligative properties
Colligative properties are the properties of
solutions that depend on the TOTAL
concentration of solute particles in solution.
The list of colligative properties includes:
a) lowering vapor pressure above a solution;
b) freezing temperature depression;
c) boiling temperature elevation;
d) osmotic pressure.
These properties depend only on the TOTAL CONCENTRATION OF ALL THE SOLUTE PARTICLES IN THE SOLUTION and completely ignore the chemical origin of solute species.
Phase diagram for a pure solvent
Pressure, P(atm)
Melting-freezing curve Liquid -vapor curve of pure solvent of pure solvent
- 00 atm
SOLID LIQUID
Freezing point Boiling point of pure solvent of pure solvent VAPOR 0 0 T^0 F T^0 B T,K
All colligative properties are based on the
lowering of the vapor pressure above the
liquid state after dissolution of solute
particles in the solvent.
Main formulas for colligative property effects:
∆ Tboiling = (TB ̶ T^0 B ) = Kb m i (1)
∆ Tfreezing = (TF ̶ T^0 F ) = ̶ KF m i (2)
- New unit of concentration: Molality, m
Molality, m = #solute moles in solution / Mass of solvent (kg)
- KF and Kb – are positive boiling and freezing constatnts for the solvent used in the experiment.
- Don’t forget: ∆ T (^) boiling = (TB ̶ T^0 B ) > 0 (because TB > T (^0) B )
∆ T (^) freezing= (TF ̶ T (^0) F ) < 0 (because TF < T^0 F )
- i – van’t Hoff factor (takes into account the dissociation of ionic solute molecules to ions)
For non-dissociative molecules (as urea, sucrose) i =1. For ionic solutes i = number of ions the solute molecule dissociates to in solution.
Examples:
For NaCl: i = 2
For urea, CO(NH2) 2 : i = 1
For Fe(NO 3 ) (^) 3: i = 4
Experimental set up
Experimental technique:
- a) Thermometer’s alignment; b) The position of the thermometer’s tip in the cooling sample.
(^0) C
Thermometer
Rubber stopper
Test tube with a liquid sample
The thermometer should have a straight central alignment, with the central position of the thermometer’s tip inside the liquid sample.
- For the freezing experiment the entire liquid sample in the test tube should be below the surface of the cooling mixture. (^0) C
Surface of the cooling mixture
Beaker with a cooling mixture Liquid sample in the test tube
Typical experimental cooling curves
Temperature a b
T^0 F a c TF
b c
0 0 Time Temperature versus time plots for a pure solvent ; for a solution
T^0 F - the freezing point temperature of the pure solvent. TF - the freezing point temperature of the solution a-line: Cooling liquid; b-line: Freezing liquid; c-line: Cooling ice
∆ Tfreezing = (TF ̶ T^0 F )
