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Properties of Solutions

Properties of solutions
When solutes are introduced in a solvent, the resulting solution differs from the initial solvent in several ways. The presence of one or more solutes alters the ability of the solvent molecules to interact and reduces their freedom of movement. This affects the solvent’s ability to move from solid to liquid or from liquid to gas. These changes are collectively referred to as colligative properties and are dependent on the total number of particles present in the solution. The four basic colligative effects due to the presence of one mole of solute per kg solvent are described in the following:

Colligative Properties:
Boiling Point
Freezing Point
Osmotic Pressure
Vapor Pressure

effects

Change per mole solute per kg solvent:
Boiling Point Elevation 0.52°C
Freezing Point Depression 1.86°C
Osmotic Pressure Elevation 17,000 mm Hg
Vapor Pressure Depression 0.3mm Hg

For a simple chemical, such as urea, the effect is related to the total number of moles of urea in solution. For a chemical compound which can dissociate, such as sodium chloride, both the sodium and the chloride ions will contribute to these colligative properties. Therefore one mole of sodium chloride will have twice the effect as will one mole of urea. Using freezing point depression as an example:
 

One mole of urea will depress the freezing point of the solution by 1.86° C

One mole of sodium chloride NaCl (1mole Na+ + 1 mole Cl-) will depress the freezing point by 3.72° C

One mole of calcium chloride CaCl2 (1 mole Ca++ + 2mole Cl-) will depress the freezing point by 5.58° C

The actual mass of the particle is irrelevant, since a small molecule will exert the same effect as a large molecule.