The following are key terms relevant to the science of osmolality and will help you to develop a better understanding of osmometry.

Avogadro’s Number: Number of molecules in one mole (gram molecular weight) of a substance. One mole of non-ionic solute (such as sucrose) dissolved in one kilogram of water will yield Avogadro’s number (6.02 x1023) of molecules. One mole of ionic solute dissolved in one kilogram of water will yield almost twice Avogadro’s number of particles.

Colligative Properties: Properties of solutions that depend on the number of particles in a given volume of solvent, not on the mass of the particles. Colligative properties include: vapor pressure, boiling point, freezing point and osmotic pressure (see also concentrative properties).

Concentration: Relative amount of solute in a solution. This can be expressed in many ways: solute to solvent, solute to solution, mass to mass, mass to volume, etc.

Concentrative Properties: When a solute is dissolved in solvent, certain properties of the solvent ―freezing point, boiling point, vapor pressure and osmotic pressure ― are changed nearly in proportion to the concentration of the solute, expressed in dissolved particles. Avogadro’s number of particles, regardless of their size or shape, when dissolved in a kilogram of water, will change each of the concentrative properties a specific amount.

Freezing Point Osmometers: Determine the osmotic strength of solution by using freezing point depression.

Freezing Point Depression: Describes the phenomenon that the freezing point of a liquid (a solvent) is depressed when another compound is added, meaning that a solution has a lower freezing point than a pure solvent.

Ionic Solution: Certain molecules, when dissolved, dissociate into charged particles called ions. A good example is sodium chloride, which dissociates in solution to sodium ions and chloride ions.

Membrane Osmometers: Measure the osmotic pressure of a solution separated by a semi-permeable membrane.

Molality: Molal concentration – grams of solute per kilogram of solvent.

Molarity: Molar concentration – grams of solute per liter of solution.

Molecular Weight: The sum of the atomic weights of all the atoms in a molecule.

Mole: Gram molecular weight, molecular weight expressed in grams. Each mole contains Avogadro’s number (6.02 x1023) of molecules. One mole of sodium chloride weighs 58.44 grams.

Non-Ionic Solution: Certain molecules, when dissolved, do not dissociate or ionize into charged particles. Good examples are glucose and urea.

Osmol: Standard unit of osmotic pressure based on a one molal concentration of an ion in a solution.

Osmolarity: Osmoles of solute per liter of solution (temperature dependent).

Osmolality: Osmoles of solute per kilogram of solvent.

Osmolality Equation: Osmolality is the number of Osmols of solute particles per kilogram of pure solvent. Since most ionic species do not completely dissociate, osmolality is a unit of concentration, which takes into account the dissociative effect. Osmolality is usually expressed in mOsm/kg H20. One milliosmol (mOsm) is 10-3 osmols. The osmolality equation is:

Osmolality = ΦnC = osmol / kg H20

Φ = osmotic coefficient, which accounts for the degree of molecular dissociation
n = number of particles into which a particle can dissociate
C = molal concentration of the solution

Osmotic Pressure: Hydrostatic pressure produced by a difference in concentration between solutions on the two sides of a surface such as a semi-permeable membrane.

Solutions: Homogeneous mixture of solutes in a solvent.

Solvent: Major liquid component of a solution.

Solutes: Minor components of a solution – usually solids.

Vapor Pressure Osmometers: Determine the concentration of osmotically active particles that reduce the vapor pressure of the solution.