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      Osmotic Pressure

The molarity of a solution is the number of moles of solute per litre of solution, in contrast to molality, which is the number of moles of solute per kilogram of pure solvent. With water as the solvent (which has a density of 1 kg per litre) and with dilute solutions, in which the change in volume of solvent upon adding solute is small, the difference between molarity and molality also is small.

In the above expression, if the osmotic pressure (P ) is measured in atmospheres, concentration (CA) is in moles per litre, and T is the absolute temperature in degrees Kelvin, then R is the gas constant encountered first in Chapter 2: R = 0.0821 litre atm deg-1mol-1. (Notice the similarity between the osmotic pressure law for ideal dilute solutions and the gas law for ideal gases.) Osmotic pressure is more sensitive to concentration than is freezing point depression, and therefore is more useful for molecular-weight determinations of large molecules.

Example. A 200-mg sample of cytochrome c is dissolved in 10 ml of water. The molecular weight of cytochrome c is 12,400. What will be the osmotic pressure in the solution when diffusion equilibrium is restored?


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