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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.
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
Boiling point elevation
- Boiling Pointtotal = Boiling Pointsolvent + ΔTb
- ΔTb = molality * Kb * i, (Kb = ebullioscopic constant, which is 0.51 K kg/mol for the boiling point of water; i = Van 't Hoff factor)
Freezing point depression
Freezing Pointtotal = Freezing Pointsolvent - ΔTf
- 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.
- πV = nRTi
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. de:Kolligative Eigenschaft hu:Kolligatív sajátság sv:Kolligativa egenskaper