Introduction:

Colligative properties are essential concepts in solution chemistry that depend solely on the number of solute particles in a solution. They play a critical role in understanding phenomena like boiling point elevation and osmotic pressure, which further aid in determining the molar mass of solutes. These principles are widely applied in both academic and real-world scenarios, making them fundamental for students in Class 12 Chemistry.

Colligative Properties:

Colligative properties are independent of the type of solute and rely only on the number of solute particles in the solution. They include:

Relative Lowering of Vapor Pressure (RLVP):

- When a non-volatile solute is added to a solvent, the vapor pressure of the solvent decreases.

- Formula:

(p0 - ps) / p0 = nB / nA

Elevation of Boiling Point:

- Adding a solute increases the boiling point of the solvent.

- Formula:

ΔTb = Kb · m

Depression of Freezing Point:

- Adding a solute lowers the freezing point of the solvent.

- Formula:

ΔTf = Kf · m

Read Also: Factors Influencing the Rate of a Reaction—Class 12 Chemistry Notes

Osmotic Pressure (π):

- Osmosis is the movement of solvent through a semipermeable membrane from a dilute to a concentrated solution.

- Formula:

π = iCRT

Determination of Molar Mass Using Colligative Properties:

From RLVP:

MB = (MA · wB · p0) / (wA · (p0 - ps))

From Elevation of Boiling Point:

MB = (1000 · Kb · wB) / (ΔTb · wA)

From Depression of Freezing Point:

MB = (1000 · Kf · wB) / (ΔTf · wA)

From Osmotic Pressure:

MB = (wB · R · T) / (π · V)

Where:

- MB: Molar mass of solute

- MA: Molar mass of solvent

- wB, wA: Masses of solute and solvent

- V: Volume of solution in liters

Van’t Hoff Factor (‘i’):

The Van’t Hoff factor accounts for the dissociation or association of solute particles.

- i > 1: For strong electrolytes (dissociation)

- i < 1: For solutes undergoing association

- Formula:

i = Observed colligative property / Calculated colligative property assuming no dissociation/association

Conclusion:

Colligative properties are pivotal in understanding solution behavior and calculating the molar mass of solutes. Mastering these concepts equips students with a strong foundation for both theoretical and practical applications in chemistry.