Question

How do you calculate osmolarity from molarity?

Answer 1

We all know that molarity is defined as the amount of moles of a compound dissolved in an amount of solvent. It is expressed in units $mol\,{L^{ - 1}}$.
$M = \dfrac{n}{V}$
Where n is the number of moles of solvent in moles
V is the volume of the solution in litres
On the other hand, osmolarity is defined as the number of osmoles of solute particles present per litre of the solution. It is expressed in units $osmol\,{L^{ - 1}}\,or\,Osm\,{L^{ - 1}}$

Complete step-by-step answer:
It is quite clear that molarity measures the number of moles of solute per unit volume of solution, while osmolarity measures the number of osmoles of solute particles per unit volume of solution.
We all know that the unit of concentration in molarity is mole. In the same way, the unit of osmotic concentration is the osmole.
An osmole is 1 mole of any fully dissociated substance dissolved in water.
For example, a 1 mole of NaCl in solution produces 2 osmoles of solute particles. In other words, we can say that a 1 $mol\,{L^{ - 1}}$ NaCl solution is a 2 $osmol\,{L^{ - 1}}\,or\,Osm\,{L^{ - 1}}$ NaCl solution.
$M = \dfrac{n}{V}$
$Osmolarity\, = \dfrac{{no.\,of\,osmoles}}{V}$
Relation between osmoles and moles of solute:
Osmoles = No. of particles solute produce in solution $\times$ no. of moles
No. of particles solute produce in solution for a NaCl solution =2
No. of particles solute produce in solution for a $N{a_2}S{O_4}$solution =3
We can write this,
$Osmolarity\, = \,Molarity \times \,no.\,of\,particles\,solute\,produce\,in\,solution$.

Note: Osmolarity and molarity are both concentration terms. The number of particles that solute produces in solution has to be calculated carefully. The relation between molarity and osmolarity and all other concentration terms should be understood properly. Osmolarity helps in the measurement of the osmotic pressure of a solution.