Osmotic Pressure Equation - Definition, Formula, Applications, FAQs

Osmosis and Osmotic Pressure

Osmosis: It is the flow of solvent molecules from a solution of low concentration to a solution of higher concentration when they are separated by a semi-permeable membrane(SPM), the concentration obviously being defined with respect to the solute.

Semi-permeable membrane consists of a network of submicroscopic pores or holes. The pore size is such that the smaller solvent molecules can move across the membrane while the movement of larger solute molecules is hindered by the smaller pores of the SPM.

There are many phenomena which include the process of osmosis that we observe in daily lives. For example, raw mangoes shrivel when pickled in brine (saltwater); wilted flowers revive when placed in freshwater, blood cells collapse when suspended in saline water, etc.

Assume that only solvent molecules can pass through these semipermeable membranes. If this membrane is placed between the solvent and solution as shown in figure given below, the solvent molecules will flow through the membrane from pure solvent to the solution. This process of flow of the solvent is called osmosis.

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The flow will continue till the equilibrium is attained. This flow of the solvent molecules to the solution side across a semipermeable membrane can be stopped if some extra pressure is applied on the solution. This pressure that just stops the flow of solvent is called osmotic pressure of the solution. The osmotic pressure is represented by the letter $\Pi$ (Pi).

This is illustrated in Figure given below. The osmotic pressure of a solution is the excess pressure that must be applied to a solution to prevent osmosis, i.e., to stop the passage of solvent molecules through a semipermeable membrane into the solution.

Osmotic pressure is a colligative property as it depends on the number of solute molecules and not on their identity. For dilute solutions, it has been found experimentally that osmotic pressure is proportional to the molarity, C of the solution at a given temperature T. Thus:

Π=CRT

Here П is the osmotic pressure and R is the gas constant. The above equation can also be written as

Π=(n2V)RT

Here V is the volume of a solution in litres containing n2 moles of solute.

If w₂ grams of solute, of molar mass, M₂ is present in the solution, then n2=w2/M2 and we can write,

ΠV=W2RTM2

Thus M2=W2RTΠV

Thus, knowing the quantities w₂, T, П and V we can calculate the molar mass of the solute.

Related Topics Link,

• Mixtures
• Solute
• Solution Properties Concentration
• Relation Between Molarity And Molality
• Ideal Solution

Recommended topic video on(osmotic pressure)

Some Solved Examples

Example.1

1. In osmosis reaction, the volume of solution:

1)Decreases slowly

2) (correct)Increases slowly

3)Suddenly increases

4)No change

Solution

Osmosis -
The spontaneous flow of solvent molecules from pure solvent to the solution through the semi-permissible membrane is known as osmosis.
Osmosis reaction takes place when the volume increases.

Hence, the answer is the option (2).

Example.2

2. If the molecular weight of the compound is increased then sensitivity is decreased in which of the following methods:

1)Elevation in boiling point

2)Viscosity

3)Osmosis

4) (correct)Dialysis

Solution

Osmosis -
The spontaneous flow of solvent molecules from the pure solvent to the solution through the semi-permeable membrane is known as osmosis.
According to the dialysis process, molecular weight increases but sensitivity decreases.

Hence, the answer is the option (4).

Example.3

In osmosis:

1)Solvent molecules move from higher concentration to lower concentration

2) (correct)Solvent molecules move from lower to higher concentration

3)Solute molecules move from higher to lower concentration

4)Solute molecules move from lower to higher concentration

Solution

In osmosis, solvent molecules move from a solution of lower concentration to a solution of higher concentration.

Hence, the answer is the option (2).

Example.4

4. Osmotic pressure is 0.0821 atm at a temperature of 300K . Find concentration in mole/liter

1)0.033

2)0.066

3) (correct)0.33×10−2

4)3

Solution

π=CRT,C=πRT=0.08210.821×300=0.33×10−2

Hence, the answer is the option (3).

Example.5

5. Osmotic pressure of a solution containing 0.1 moles of solute per liter at 273 K is (in atm)

1) (correct)2.24

2)22.4

3)4.48

4)44.8

Solution

π=CRTπ=wm×RT=0.11×0.0821×273=2.24

Hence, the answer is (2.24).

Example.6

6. If 3 gm of glucose (mol. wt. 180) is dissolved in 60 gm of water at 15⁰C . Then the osmotic pressure (in atm ) of this solution will be

1) (correct)6.57

2)0.34

3)0.65

4)5.57

Solution

π=CRT=3×1000180×60×0.0821×288=6.56 atm

Hence, the answer is (6.56 atm).

Summary

Osmotic pressure is very beneficial as it has various applications in various fields such as it has various applications in the medical field, in biology and medicine, also in the food industry, in industrial processes. In the medical field dialysis is the main and very important aspect of it in this the osmotic pressure is utilized to remove the waste product from the blood of a patient with kidney failure. It is also important to maintain the isotonic Solutions means the balance of fluid in the body of the patient. Reverse osmosis is very important and used to desaline the water means to purify the water by pushing a semipermeable membrane removing salt and contaminants. Also check-

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NCERT Chemistry Notes:

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