Foreword
15
OsmoTECH XT Single-Sample Micro-Osmometer User Guide
Definitions
Following are key terms relevant to the science of osmolality.
Avogadro’s number
The 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 (6.022 x 10
23
) of molecules. One mole of
ionic solute dissolved in one kilogram of water will yield almost twice Avogadro’s number of particles.
Colligative properties
The properties of solutions depend on the number of particles in each volume of solvent, not the mass of the particles.
Colligative properties include: vapor pressure, boiling point, freezing point and osmotic pressure (see also
Concentrative Properties).
Concentration
The 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
Instruments which measure the osmotic strength of solution by using freezing point depression.
Freezing point depression
The phenomenon where the freezing point of a liquid (a solvent) is depressed when another compound is added,
meaning 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
(NaCl), which dissociates in solution to sodium ions (Na+) and chlorine (Cl-) ions.
Membrane osmometers
Instruments which 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.
Mole
Molecular weight expressed in grams. Each mole contains Avogadro’s number (6.022 x 10
23
) of molecules. One mole
of sodium chloride weighs 58.44 grams.
Molecular weight
The sum of the atomic weights of all the atoms in a molecule.
