abc osmolarity

HOW DO YOU CALCULATE OSMOLARITY?

When in solution, some solutes have molecules which dissociate into several particles.

For example,

- potassium chloride molecule (KCl) dissociates into two particles ( K⁺, Cl^--),
- sodium chloride molecule (NaCl) also dissociates into two particles (Na⁺, Cl^-),
- calcium chloride molecule (CaCl₂) dissociates into three particles (Ca⁺, Cl^-, Cl^-).

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

osmolarity = molarity x n
n: number of particles that dissociated from the solute molecule. For example, the osmolarity of a 2M solution of NaCl is: 2x2 = 4 osmol/L;
the osmolarity of a 1M solution of CaCl₂ is 1x3 = 3 osmol/L.
This equation can be also used to calculate the osmolarity of solutions whose solutes that do not dissociate such as glucose, urea, glycerol, .... Their osmolarity equals their molarity because n=1.
The osmolarity of a 4M solution of glucose is 4x1 = 4 osmol/L.

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 reassociate with each other once dissolved.

Helas! as you know, the REAL world is quite different from the IDEAL one.

REAL solutions do not quite behave like ideal solutions: for example, in real solutions, some cations and anions of a dissolved electrolytes will reassociate at a given time.

This difference between IDEAL and REAL solutions means that the true (or measured) osmolarity of the solution is different than the predicted osmolarity calculated with the equation above.

The true osmolarity of a solution can be determined from its colligative properties (by measuring the solution osmotic pressure, freezing point, boiling point or solvent vapor pressure). For a given solution:

TRUE osmolarity / PREDICTED osmolarity = f
f is the osmotic coefficient

Researchers have determined TRUE osmolarity and calculated the osmotic coefficient for various solutions. They have listed their finding in tables that can found in Chemical Handbooks.

Value of the osmotic coefficient depends 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 (as solutions becomes more dilute they behave more and more like IDEAL solutions).

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.

The TRUE osmolarity of solutions containing single solutes 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

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.

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