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Osmolarity vs. Osmolality

Difference between Osmolarity and Osmolality
Measurements of osmolar concentration are often expressed in osmolarity and osmolality. Osmolarity is a measure of the osmoles of solute per liter of solution. A capital letter M is used to abbreviate units of mol/L. Since the volume of solution changes with the amount of solute added as well as with changes in temperature and pressure, osmolarity is difficult to determine.

Osmolality is a measure of the moles (or osmoles) of solute per kilogram of solvent expressed as (mol/kg, molal, or m). Since the amount of solvent will remain constant regardless of changes in temperature and pressure, osmolality is easier to evaluate and is more commonly used, and often preferred, in practical osmometry. Most commercially available osmometers report results using osmolality units mOsm/kg. 

As a general rule,  a 1 molar solution will have a higher solute concentation than a 1 molal solution. The reason for this is that the solute takes up some volume in the total solution. Consequently, the freezing point depression of a 1 M solution will be lower when compared to the same 1 molal solution. The difference between the freezing point determination will be more noticeable at higher concentrations (i.e 2M, 3M, 4M) and will be alsmost negligable at more dilute concentrations, say 0.5 M or lower.