HOW DO YOU CALCULATE OSMOLARITY?

The osmolarity of solutions containing single solutes can be calculated from the following equation:

osmolarity = molarity x n x f
n: number of particles that dissociated from the solute molecule.
f: osmotic coefficient of the solute
Value of the osmotic coefficient depend on the concentration of the solute and its chemical properties. The osmotic coefficient may be smaller or greater than one. It is smaller than one for electrolytes and for all solutes it approaches one as the solution becomes more and more dilute.

The values of f have to be determined by experimentation (by measuring the osmotic pressure, or the freezing point, or the boiling point, or the solvent vapor pressure). This has been done by researchers and now f can be obtained from tables in Chemical Handbooks that list values of f for different substances as function of concentration.

We have listed the osmotic coefficients for the solutes encountered in the extracellular fluids of mammals. f are given for the concentrations they are found in this extracellular fluid.

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