60 Extra: Concept of Osmolality and Milliequivalent
In order to calculate osmotic pressure, it is necessary to understand how solute concentrations are measured. The unit for measuring solutes is the mole. One mole is defined as the molecular weight of the solute in grams. For example, the molecular weight of sodium chloride is 58.44; thus, one mole of sodium chloride weighs 58.44 grams. A solution’s molarity is the number of moles of solute per liter of solution. On the other hand, a solution’s molality is the number of moles of solute per kilogram of solvent. If the solvent is water, one kilogram of water is equal to one liter of water. Osmolarity is related to osmolality, but is affected by changes in water content, as well as temperature and pressure. In contrast, osmolality is unaffected by temperature and pressure.
Molarity and molality represent solution concentration, but electrolyte concentrations are usually expressed in terms of milliequivalents per liter (mEq/L). The mEq/L is the ion concentration, in millimoles, multiplied by the number of electrical charges on the ion. The milliequivalent unit incorporates both the ion concentration and the charge on the ions.
Thus, for ions that have a charge of one, such as sodium (Na+), one milliequivalent is equal to one millimole. For ions that have a charge of two, such as calcium (Ca2+), one milliequivalent is equal to 0.5 millimoles. Another unit of electrolyte concentration is the milliosmole (mOsm), which is the number of milliequivalents of solute per kilogram of solvent. Osmoregulation maintains body fluids in a range of 280 to 300 mOsm.