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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.
Additional recommended knowledge
The relationship between the lowering of vapor pressure and concentration is given by Raoult's law, which states that:
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
Freezing point depression
Freezing Pointtotal = Freezing Pointsolvent - ΔTf
where :ΔTf= molality * Kf, (Kf = constant kinetic energy, which is 1.86 for freezing point)
For a fixed mass of 1 kg of solvent, the change in temperature is given by
where m is the mass of solute, M is the mass percentage of solute, and K is the boiling-point constant or freezing-point constant for that particular solvent.
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:
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:
where: π = osmotic pressure; V is the volume containing 1 mole of solute; T is absolute temperature; n is the number of moles of solute; R = 8.3145 J K-1mol-1, the molar gas constant.
Categories: Solutions | Physical chemistry
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Colligative_properties". A list of authors is available in Wikipedia.|