Question

The Osmotic pressure of solution increases if
1. Temperature is decreased
2. Solution constant is increased
3. Number of solute molecules is increased
4. Volume is increased

Answer 1

Correct Answer - Option 3 : Number of solute molecules is increased

Concept: 

Osmosis:

• The phenomenon in which the flow of solvent through a semi-permeable membrane from the pure solvent to the solution takes place is osmosis. 
• It involves the movement of solvent molecules only.
• Osmosis is limited to solutions only.
• Osmosis takes place through a semi-permeable membrane.
• The solvent molecules move from the solution of low solute concentration to higher solute concentration.

Osmotic pressure:

• The osmotic pressure of a solution is called the excess pressure need to be applied to a solution to prevent osmosis. i.e., to stop the passage of solvent molecules through a semi-permeable membrane into the solution.
• It is denoted by π.
• Osmotic pressure is one of the colligative properties as it depends on the number of solute molecules and not on their identity.
• According to Van't Hoff, For a dilute solution, Osmotic pressure is given by:

\({\mathbf{\pi }}{\text{ }} = \;c{\mathbf{RT}}\;\)

\({\mathbf{\pi }}{\text{ }} = \;\frac{{{{\text{n}}_2}}}{{\text{V}}}{\mathbf{RT}}\;\) (\(\because c{\text{ = }}\frac{{{{\text{n}}_{\text{2}}}}}{{\text{V}}}\))

\({\mathbf{\pi }}{\text{V = }}\frac{{{{\text{w}}_{\text{2}}}{\text{RT}}}}{{{{\text{M}}_{\text{2}}}}}\;\) (\(\because {n_2}{\text{ = }}\frac{{{w_2}}}{{{M_2}}}\))

Where c = molar concentration/ molarity of the solution; T = Temperature; w₂ = mass of solute; M₂ = molecular weight of solute present in solution, R = gas constant; V = volume of solution in litres; n₂ = no. of moles of solute 

For a solution, at a given temperature, R and T are constant, then Osmotic pressure is:

\(\pi \propto c\)

Explanation:

Factors affecting Osmotic pressure is given below:

Factors	proportionality	Osmotic pressure trend
Temperature	\(\pi \propto T\)	As temperature increases, osmotic pressure increases
Gas constant	\(\pi \propto R\)	R is a constant value for all solution
No: of moles of solute	\(\pi \propto {n_2}\)	As no. of moles of solute increases, O.P increases
Volume of solution	\(\pi \propto \frac{1}{V}\)	As volume is increased, O.P gets decreased
The molecular mass of solute	\(\pi \propto \frac{1}{{{{\text{M}}_{\text{2}}}}}\)	As molar mass increase, O.P decreases
Molarity of solution	\(\pi \propto c\)	As the molarity of solution increase, O.P also increase

Hence, The Osmotic pressure of the solution increases, if the number of solute molecules is increased.