OsmoPRO® Multi-Sample Micro-Osmometer User’s Guide
23
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.
Molecular weight
The sum of the atomic weights of all the atoms in a molecule.
Mole
Molecular weight expressed in grams. Each mole contains Avogadro’s number (6.022 x10
23
) 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 / Osmole
The standard unit of osmolality based on a one molal concentration of an ion in a solution.
Osmolarity
The number of osmoles of solute per liter of solution (temperature dependent).
Osmolality
The number of osmoles of solute per kilogram of solvent (temperature independent).
Osmolality equation
Osmolality is the number of osmoles of solute particles per kilogram of pure solvent. Since most
ionic species do not completely dissociate, osmolality is a unit of concentration, which must take
into account the dissociative effect. Osmolality is usually expressed in mOsm/kg H
2
O. One
milliosmole (mOsm) is 10
-3
osmoles. The osmolality equation is:
Osmolality = ΦnC = osmole / kg H
2
O
Where:
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
The hydrostatic pressure produced by a difference in concentration between solutions on two
sides of a surface such as a semi-permeable membrane.
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