HOW DO YOU CALCULATE OSMOLARITY?
The osmolarity of solutions containing a single type of solute
(for example: just glucose or just sodium chloride) 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
The osmolarity of solutions containing many different type of
solutes (for example: glucose and sodium chloride) can be calculated
from the following equation:
osmolarity = SUM OF ALL (molarity x n x f) OF EACH SOLUTE
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.