Question

What is osmolarity?

Answer 1

Hint: The osmotic pressure of a solution is defined as the pressure required or created to establish an equilibrium across a semipermeable membrane through which a solution is passing. The osmolarity is similar to the molarity but osmolarity refers to the concentration of osmoles required to create this equilibrium.

Complete answer:
When the molecules of a solution move from a region of low concentration to high concentration across a semipermeable membrane, the process is known as osmosis. It is a passive process and does not require any energy to take place.
The pressure applied to prevent or stop the movement of a solvent across a semipermeable membrane is known as osmotic pressure.
The term that is used to describe the number of particles of a substance that contributes to the osmotic pressure of the solution is known as osmoles. It is similar to the term moles and equal to the molar mass of the substance divided by the number of particles or ions the substance dissociates into.
As we calculate molarity to find out the concentration of any solution in moles per liter, similarly we calculate the concentration of a solution undergoing the process of osmosis in terms of osmoles per liter. The term used to express the osmotic concentration is known as osmolarity which can be defined as the number of osmoles of solute per liter of solution.
$$\text{Osmolarity (Osm)}={\displaystyle \frac{\text{No}\text{. of osmoles (osmol)}}{\text{Volume of solution (in L)}}}$$
Osmolarity depends on the number of particles in a chemical solution, but not on their identity. For example, NaCl dissociates into ions and thus 1 mole of NaCl in solution contributes 2 osmoles of solute particles, that are sodium and chloride ions. Both ions will affect the osmotic pressure of the solution.

Note:
Osmolarity is preferably used when all measurements are made at a constant temperature and pressure. Because the volume of a solution changes with the temperature change and it becomes difficult to determine the accurate value of osmolarity.