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An Overview of Osmolarity

The amount of solute in a specific amount of solvent helps us determine the concentration of a substance. On the other hand, osmolarity helps in the determination of the movement of a solute across different solutions.

The concept of concentration holds importance in our daily practices as well, like adding sugar or salt to lemonade or making a strong or dilute coffee. It also finds use in biology since the chemicals being injected in our body must have the same concentration as our body fluid.

With the progress of the chapter, an understanding of the concept and uses of concentration, the process of osmosis and the factors affecting the same will be brought upon.


Osmolarity is usually associated with determining the concentration of solutes that can disassociate. It can be understood better as the number of osmoles per litre of a solution or as the concentration of a specific solute or solutes per litre of solvent. Osmolarity is best defined in terms of osmoles. The same can be mathematically expressed as

$Osmolarity = \dfrac{Number \ of \ osmoles \ present \ in \ given \ amount \ of \ solute}{Volume \ of \ solution}$ .


The osmole is a unit of measurement for osmolarity and it is the indication of the presence of a number of moles of a solute in a solution that contributes to osmotic pressure. For a solute that does not undergo dissociation, osmoles are equal to its moles. On the other hand, for solutes that dissociate into ions, osmoles depend on the moles as well as on the number of ions the solute dissociates into. For example, for 1 mole of glucose, the osmole is also 1 but for one mole of NaCl, the osmoles are 2 since NaCl is capable of dissociating into $Na^+$ and $Cl^-$.

Factors Affecting Osmolarity

Osmolarity is dependent on multiple factors. The number of moles and the number of ions a solute dissociates into also determines osmolarity. The more is the number of ions the solute dissociates into, the higher is the osmolarity. The same is valid for the number of moles as well.

Since it depends on the volume of solution, consequently, it is also dependent on temperature. As the temperature increases, osmolarity decreases due to an increase in volume.

Osmotic Pressure

The least amount of pressure to be applied to a solute in order to stop the process of osmosis is called osmotic pressure. The osmotic pressure for pure water is zero atm. It is a colligative property. These properties are dependent on the number of solute particles and independent of their nature.

Few examples of colligative properties, other than osmotic pressure, include elevation in boiling point, depression in freezing point, and relative lowering in vapour pressure. Osmotic pressure is represented by $\pi$ and depends upon osmolarity since it is directly proportional to concentration. It also depends on gas constant, temperature, and Van't Hoff’s factor.

The mathematical equation to determine osmotic pressure is given below:

$\pi \ = iRTC$;

where, i = Van’t Hoff factor,

C = Concentration of solute (mol/L),

T = Temperature of the solution and R = ideal gas constant


When two solutions, having different osmolarity, are separated by a semipermeable membrane, their solvents tend to flow under the process of osmosis. The solution containing less solute, i.e., lesser osmolarity but more amount of solvent, tends to diffuse through the semipermeable membrane towards the solution having high amount of solute, i.e., higher osmolarity but lesser amount of solvent. There is no movement in solute particles since the semi-permeable does not let through any solute particles. This can also be considered as the diffusion of solvent from its higher concentration to its lower concentration.

Significance of Osmolarity

Osmolarity has significance in chemical as well as biological fields. In Chemistry, it can affect the concentration of reactants and thus cause significant changes in the reaction product. It helps to calculate osmotic pressure and thus helps determine the movement of solvent during osmosis. This enables us to understand various biological phenomena. It is very important to determine the osmolarity of various fluids inside our body. If a fluid of higher or lower osmolarity is injected into our body, our cells will be damaged due to exosmosis or endosmosis.


Concentration is a means to help us distinguish between which solution has more amount of solute per unit of volume. One method of expressing concentration is osmolarity. It is understood as the number of osmoles per unit volume (litre). Osmoles are the number of particles of solute present in solution when osmolarity is being calculated. Osmolarity helps calculate the osmotic pressure, which is a colligative property. In turn, osmotic pressure can be used to calculate molar mass of solute. Osmolarity is affected by factors including temperature, pressure, and volume of solution. It holds significance in both chemical and biological fields.

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FAQs on Osmolarity

1. What is the difference between molarity and osmolarity?



It refers to the number of moles per unit volume in a solution.

It refers to the number of osmoles per unit volume in a solution.

Its units are $moles \ litre^-$

Its units are $osmoles \ litre^-$

Molarity of solutes having the same number of moles is same, whether it dissociates or not (Given that volume is constant).

Osmolarity of solutes having the same number of moles is the same unless one disassociates. In that case, the more are the ions generated on disassociation, the more is the osmolarity (given that volume is constant).

2. What is the difference between osmolarity and osmolality?



It refers to the number of osmoles per unit volume in a solution.

It refers to the number of osmoles per unit mass of solute.

Its units are $osmoles \ litre^-$

Its units are $osmoles \ Kilogram^-$

It is dependent on temperature changes since an increase in temperature leads to an increase in volume, and thus a decrease in osmolarity.

It is independent of temperature changes.

3. How does osmolarity affect the rate of osmosis?

The relatively older term for osmotic concentration is osmolarity. The concentration of solution determines the direction of osmosis as well as measure of osmotic pressure. The more is the volume of the solvent of a solution, the more is its tendency to move towards a solution having a lesser volume of solvent. Therefore, osmotic movement of solvent occurs from a lower concentration of solution to a higher concentration of a solution or from a higher concentration of solvent to a lower concentration of solvent.