What is Osmolarity?


In simple terms, Osmolarity is a process of measurement of solute concentration. It is the number of osmoles (Osm) of solute per litre (L) of solution (osmol/L or Osm/L).

Usually, it is written as Osm/L. This resultant number is of higher importance as it allows the measurement of the osmotic pressure of a solution and the determination of how the solvent will diffuse across a semi-permeable membrane separating two solutions of different osmotic concentration.

Also, the unit of measuring it is the osmole. It’s a non-SI unit of measurement that defines the number of moles of solute that contribute to the osmotic pressure of a solution. It is measured using an osmometer.


Expression of Osmolarity

Below is the expression to express the osmolarity of a solution, given in osmoles per litre (osmol/L).

Osmolarity = \sum_{}^{i\varphi }i\eta i Ⅽi

Where, the sign \varphi }  is the osmotic coefficient, which accounts for the degree of non-ideality of the solution. In the simple term, it is the degree of dissociation of the solute. n is the number of particles (e.g. ions) into which a molecule dissociates.

C is the molar concentration of the solute. The index i defines the identity of a particular solute.

Importance of Osmolarity

It is very important because cells cannot survive if the osmolarity if their surroundings are much different from their own. Water moves from the side of the membrane to another side: from a lower to a higher.

In other words, it changes from the dilute side to the concentrated side. It is a continuous process in which dilute side loses water and becomes more concentrated and the concentrated side gains water to become more dilute.

Cell survives by balancing water uptake and loss. Hence the process of osmosis is so important as cells can burst if they take on too much water, or collapse if they lose too much.

Osmolarity vs Molarity

Molarity of a solution is defined as the concentration of a solute in that solution. It tells you how much of a solute is present. The units of molarity are moles (mols).

Whereas it helps us to determine if water will move from one side of a membrane to the other side. That is: will Osmosis occur? Its unit is osmols (osm). We must note down the fact that water will always move across a membrane into the solution with the higher osmolarity.

Before determining the osmolarity of a solution we need to figure out its molarity first. Now before we calculate molarity we need to understand the following ideas.

We must know that 1 Gram-Molecular weight of a compound is its molecular wt. expressed in grams and that

1 Gm.-Mol. Wt. = 1 mole.  i.e., 1 mole of glucose  = 180 grams.

You will need to know the Mole Weight of the solute and the volume of the solution. You also need to know how much solute (in grams) is present in that volume of solution.

* According to Avogadro’s Law 1 mole of anything contains 6.02 x 1023  particles of that solute.

* Secondly a 1 mole of solute per litre of solution  =  a 1 molar solution  =  1M.

*  we use the following equation to calculate the molarity of a solution,

(Molarity) M  =  \frac{Amount of solute (in grams) per liter}{ Mol. Wt. of solute}\

* For calculating the osmolarity of a solution we use the following equation:

(Osmolarity) OSM  =  M  x  the number of particles of dissociation.

*  For example: Like Sodium chloride(NaCl) in water dissociates into two particles (Na+ and Cl) and Magnesium chloride(MgCl2) in water dissociates into 3 particles (Mg++ and 2 Cl).

Solved Questions for You

 Q.1. What does blood osmolarity mean?

 Ans- Plasma osmolarity: The osmolarity of blood plasma, which is a measure of the hydration status. It is a lot sensitive to changes in hydration status during dehydration and rehydration. The normal range of it is between 280-300 mos/kg.

Q.2. Does high osmolarity mean more water?

Ans- Water has a tendency to move across a membrane from a lower osmolarity to a higher. In simple words, from the dilute side to the concentrated side. Thus yes, water goes from low to high, from weak to concentrated solutions.

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