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 osmolarity of a REAL solution is slightly different from its IDEAL equivalent calculated with the equation above.

REAL SOLUTION: real (= measured) osmolarity – is measured with an osmometer

IDEAL SOLUTION: ideal (= theoretical) osmolarity – is calculated with the equation above.

REAL osmolarity / IDEAL osmolarity = f
f is the osmotic coefficient of the solution

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 (= ideal) colligative properties with the real (= 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)

The value of the osmotic coefficient of a solution depends on many factors such as the concentration of the solutes present; the types of solute presents; temperature, etc…

To illustrate, we have listed the osmotic coefficients of solutions – Each solution listed contains only one type of solute and the solute concentration is the same as what is found in the extracellular fluids of mammals - the temperature of all the solutions is 37^oC.

Solutions containing single solutes:

IDEAL osmolarity = molarity x n

If f is given to you, you can calculate their REAL osmolarity:

REAL osmolarity = IDEAL osmolarity x f
REAL osmolarity = molarity x n x f
n: number of particles that dissociated from the solute molecule.
f: osmotic coefficient of the solute (can be found on tables)

Solution containing several solutes:

IDEAL OSMOLARITY = SUM OF ALL (molarity x n) OF EACH SOLUTE

(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)

The REAL OSMOLARITY could be calculated using this equation:

REAL osmolarity = SUM OF ALL (molarity x n x f) OF EACH SOLUTE.

In practice it is quite impossible to determine f for each individual solute within this solution that is made of many type of solute. Therefore it is best to measure the REAL osmolarity using an osmometer.