Everything about Colligative Property totally explained
Colligative properties are properties of solutions that depend on the number of particles in a given volume of solvent and not on the mass of the particles. Colligative properties include: lowering of
vapor pressure;
elevation of boiling point;
depression of freezing point;
osmotic pressure (see
Osmosis;
Reverse Osmosis). Measurements of these properties for a dilute aqueous solution of a non-ionized solute such as
urea or
glucose can lead to accurate determinations of relative molecular masses. Alternatively, measurements for ionized solutes can lead to an estimation of the percentage of ionization taking place.
Vapor pressure
The relationship between the lowering of vapor pressure and concentration is given by
Raoult's law, which states that:
» The vapor pressure of an ideal solution is dependent on the vapor pressure of each chemical component and the mole fraction of the component present in the solution. (For details, see the article on Raoult's law.)
Boiling point and freezing point
Both the
boiling point elevation and the
freezing point depression are proportional to the lowering of vapor pressure in a dilute solution
Boiling point elevation
» Boiling Point
total = Boiling Point
solvent + ΔT
b
where
» ΔT
b =
molality * K
b *
i, (K
b = ebullioscopic constant, which is 0.51 K kg/mol for the boiling point of water;
i =
Van 't Hoff factor)
Freezing point depression
Freezing Point
total = Freezing Point
solvent - ΔT
f
where :ΔT
f =
molality * K
f *
i, (K
f = cryoscopic constant, which is 1.86 K kg/mol for the freezing point of water;
i =
Van 't Hoff factor)
Osmotic pressure
Two laws governing the osmotic pressure of a dilute solution were discovered by the German botanist
W. F. P. Pfeffer and the Dutch chemist
J. H. van’t Hoff:
- The osmotic pressure of a dilute solution at constant temperature is directly proportional to its concentration.
- The osmotic pressure of a solution is directly proportional to its absolute temperature.
These are analogous to
Boyle's law and
Charles's Law for gases. Similarly, the combined
ideal gas law, PV = nRT, has an analog for ideal solutions:
» πV = nRT
i
where: π = osmotic pressure; V is the volume; T is absolute temperature; n is the number of moles of solute; R = 8.3145 J K-1mol-1, the molar
gas constant;
i =
Van 't Hoff factor.
Further Information
Get more info on 'Colligative Property'.
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