By using
this equation, we assume that we are working with IDEAL solutions. In ideal solutions,
solutes molecules dissociate in predicted fashions and solute particles do not
re-associate with each other once dissolved.
As you know, the REAL
world is usually quite different from the IDEAL one.
REAL
solutions do not quite behave like ideal solutions: for example, in real
solutions, there is a statistical probability that some cations and anions of a
dissolved electrolytes can re-associate temporarily at a given time.
This
difference between IDEAL and REAL solutions means that the true (or measured) osmolarity of the solution is slightly
different than the ideal (or predicted) osmolarity calculated with the equation
above.
TRUE
osmolarity / PREDICTED osmolarity = f
f is the osmotic coefficient
The osmotic coefficient (f) of a solution can be
determined from its colligative properties.
The
colligative properties of a solution are
- the properties that depend on the total number of solute particles in
a given volume, regardless of their chemical types;
- a solution osmotic pressure, depression of the freezing point, elevation of the boiling point and depression of the water
vapor pressure.
By comparing the solution’s
theoretical colligative properties with the true (or measured) colligative
properties, we can determine the osmotic coefficient of a solution (f). For example: the theoretical freezing point depression (∆Tf) of a solution can be
calculated (Ideal ∆Tf) as well as measured (Real ∆Tf) and f = (Real
∆Tf) / (Ideal ∆Tf)
Value of the osmotic coefficient depends on many factors such as the concentration of the solute; the solute type; temperature,
etc… To illustrate, we have listed the osmotic
coefficients for the solutes encountered in the extracellular fluids of
mammals. f
are given for the concentrations and temperature (37oC) these
solutes are found in the extracellular fluids of mammals.
Solutions containing
a single solute type ONLY (for example a solution of NaCl
only):
The
TRUE osmolarity of solutions containing single solute type can be calculated
from the following equation:
TRUE
osmolarity = PREDICTED osmolarity x f
TRUE osmolarity = molarity
x n x f
n: number of particles that
dissociated from the solute molecule.
f: osmotic coefficient of the solute
Solutions containing
several solute type ONLY (for example a solution of NaCl
and glucose):
It
is possible to calculate the PREDICTED OSMOLARITY of an IDEAL SOLUTION
containing several solutes (e.g. -
for an ideal solution of 0.3M of glucose, 0.2M NaCl and 0.1M CaCl2, -
the osmolarity is (0.3 x 1) + (0.2 x 2) + (0.1 x 3) = 1.0 Osm)
However, The TRUE OSMOLARITY of a
REAL SOLUTION containing several solutes has to be determined from its
colligative properties.