Osmolarity and osmolality are different ways of representing the concentration of a solute in a solution. Defining individually, osmolarity denotes the number of solute particles in one liter of the solvent, just like the molar concentrations.

On the other hand, osmolality refers to an amount of solute particles as per one kilogram of the solvent i.e., the number of particles/weight of the solvent.

Let us now try to tackle and understand some major differences between the two and how those differences might have an effect on the different properties associated with them.

Units

The units in which they osmolarity and osmolality can be represented are in correspondence with their definitions too. For instance, the concentration of solute particles in a solution is called as osmoles written in abbreviation as ‘Osm’.

Accordingly, the units representing osmolarity and osmolality are osmoles/L or Osm/L and osmoles/kg or Osm/kg, respectively. Similarly, these concentrations can be easily converted into the units in which the concentration is being considered e.g., milliosmoles or micro osmoles – for describing the physiological concentrations of some solutes.

Calculation of Osmolarity and Osmolality

In most of the calculations of the amounts of solutes in solvents, the number of moles is used. A mole (mol) or molar concentration of a chemical compound represents the number of molecules of that particular substance, which is also very specific to that substance only.

For example, different amounts of NaCl and CaCl­2 would make their 1 molar solutions which is in correspondence to their molecular weights. But when talking about osmoles, it depends on how much ions a molecule can contribute and then multiply this with the molar concentration will give the number of osmoles.

Let’s try to understand this with simple examples and calculations. If we have 1 M (M = molar) solution of NaCl (molecular weight of NaCl / Liter) we cannot say that it is also 1 osmole of NaCl. This is because in water NaCl dissociates to form 2 types of ions – Na+ and Cl-. Therefore the solution would have 1 mole of Na+ and I mole of Cl, adding together to make 2 osmoles in the solution.

Now let’s see how osmolality would be calculated. After we have got an idea of calculating the osmolarity and a general principle to calculate it, it is now easier for us to calculate osmolality.

If we take NaCl, and we know that it contributes two osmoles in a solution, then logically the amount of NaCl measured per kg of the solution would give us its osmolality. For instance, if we have 1 molar solution of NaCl and adjust the volume of the solution to kg, we would have a solution containing 2 osmoles of NaCl per kg of solvent.

Temperature Dependence of Osmolar and Osmolal solutions

With the changes in temperatures, the volumes of the solvents change accordingly. Let’s take a simple example. When the temperature of water is increased its volume also is increased. Contrastingly, when the temperature is decreased water’s volume is decreased.

Therefore, since osmolarity od dependent on the volume of the solutes, with changing temperatures and thus changing volumes, its amount will be affected. Similarly, other colligative properties are also affected in osmolar solutions.

Quite contrastingly, as osmolality is measured per kg of the solvent and that this weight is independent of the effects of changing temperatures, osmolality is also independent of the changes in temperatures. This is also the reason why physiological measurements in terms of osmolality are preferred in biological systems over osmolarity.

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