Osmometry and Measuring the Change in Freezing Point, Boiling Point and Vapor Pressure with Osmolality

 

A 1 osmolal solution of a pure, nondissociated solute in 1 kilogram of water contains Avogadro's number of particles (6.0225 * (10 ^23)).

 

In practice, the osmolality of a solution is affected by a number of factors, including:

(1) the number of different solutes present

(2) the concentration of each solute present

(3) the number of particles the solute dissociates into when in solution

(4) associations between solute particles

(5) associations between solute and solvent

 

osmolality of an aqueous solution =

= (osmotic coefficient for the solute) * (number of particles which each solute molecule potentially dissociates into when in solution) * (concentration of solute in mole per kg water)

 

The physical properties of the solution are altered by its osmolality. For water, changes in freezing point, boiling point and vapor pressure can be measured.

Property	Normal Finding of Pure Water	Effect of Solute	1 osmol solute per kg water
freezing point	0° C	decreased	1.858° C
boiling point	100° C	increased	0.52° C
vapor pressure	23.8 mm Hg at 25°C	decreased	0.3 mm Hg

 

Measuring Osmolality of a Solution

 

milliosmol per kg water =

= ((freezing point of solution) / (-1.858)) * 1000

= (((boiling point of water) - 100) / 0.52) * 1000

= (((23.8 mm Hg at 25°C) - (vapor pressure at 25° C)) / 0.3) * 1000

 

Measuring Change in Physical Parameters

 

freezing point of solution in °C =

= ((mOsm per kg water) / 1000) * (-1.858° C)

 

boiling point of solution in °C =

= 100 + ((mOsm per kg water) / 1000) * (0.52° C)

 

vapor pressure of solution in mm Hg =

= 23.8 - (((mOsm per kg water) / 1000) * (0.3))