Colligative Properties

topicmedium9 MCQ

Relative lowering of vapour pressure, boiling point elevation, freezing point depression and osmotic pressure. (Chemistry › Solutions, NEET UG syllabus.)

Practice 9 questions Back to syllabus~14 min

What is Colligative Properties?

Properties of solutions that depend only on the number of solute particles present, irrespective of their nature.

Key formula / rule: Relative Lowering of Vapour Pressure (Raoult's Law)

Key points

Define colligative properties and list the four main types.
Explain the molecular basis for each colligative property.
State and apply Raoult's Law for relative lowering of vapour pressure.
Calculate boiling point elevation and freezing point depression using appropriate formulae.

Common exam trap

Forgetting to use the van't Hoff factor (i) for electrolytes.

Definitions

Term: Colligative Properties
Meaning: Properties of solutions that depend only on the number of solute particles present, irrespective of their nature.

Term: Relative Lowering of Vapour Pressure
Meaning: The fractional decrease in the vapour pressure of a solvent when a non-volatile solute is added, equal to the mole fraction of the solute.

Term: Boiling Point Elevation
Meaning: The increase in the boiling point of a solvent when a non-volatile solute is dissolved in it, compared to the pure solvent.

Term: Freezing Point Depression
Meaning: The decrease in the freezing point of a solvent when a non-volatile solute is dissolved in it, compared to the pure solvent.

Term: Osmotic Pressure
Meaning: The pressure that must be applied to a solution to prevent the inward flow of solvent across a semi-permeable membrane.

Term: Semi-permeable Membrane (SPM)
Meaning: A membrane that allows certain molecules or ions to pass through it by diffusion or osmosis, typically solvent molecules but not solute molecules.

Term: Van't Hoff factor (i)
Meaning: A factor that accounts for the dissociation or association of solute particles in a solution, modifying the colligative property equations.

Term: Isotonic Solutions
Meaning: Solutions having the same osmotic pressure at a given temperature.

Term: Hypotonic Solution
Meaning: A solution with a lower osmotic pressure compared to another solution, causing solvent to move into the other solution.

Term: Hypertonic Solution
Meaning: A solution with a higher osmotic pressure compared to another solution, causing solvent to move out of the other solution.

Learning objectives

Define colligative properties and list the four main types.
Explain the molecular basis for each colligative property.
State and apply Raoult's Law for relative lowering of vapour pressure.
Calculate boiling point elevation and freezing point depression using appropriate formulae.
Calculate osmotic pressure and understand its significance.
Incorporate the van't Hoff factor (i) in colligative property calculations for electrolytes.
Determine the molar mass of an unknown solute using colligative properties.
Differentiate between isotonic, hypotonic, and hypertonic solutions.

Formulae

Name: Relative Lowering of Vapour Pressure (Raoult's Law)
Note: P° is vapor pressure of pure solvent, P_s is vapor pressure of solution, x_solute is mole fraction of solute.
Expression: (P° - P_s) / P° = i * x_solute

Name: Elevation in Boiling Point
Note: ΔT_b = T_b(solution) - T_b(solvent). K_b is molal elevation constant (ebullioscopic constant), m is molality of solute.
Expression: ΔT_b = i * K_b * m

Name: Depression in Freezing Point
Note: ΔT_f = T_f(solvent) - T_f(solution). K_f is molal depression constant (cryoscopic constant), m is molality of solute.
Expression: ΔT_f = i * K_f * m

Name: Osmotic Pressure
Note: π is osmotic pressure, C is molarity of solution, R is gas constant (0.0821 L atm mol⁻¹ K⁻¹ or 8.314 J mol⁻¹ K⁻¹), T is temperature in Kelvin.
Expression: π = i * C * R * T

Name: Van't Hoff factor (i)
Note: i = 1 for non-electrolytes. For electrolytes, i > 1 (dissociation), i < 1 (association).
Expression: i = (Observed colligative property) / (Calculated colligative property assuming no association/dissociation)

Prerequisites

Basic understanding of solutions, solvents, and solutes.
Knowledge of concentration terms: molarity, molality, mole fraction.
Concept of vapour pressure and phase transitions (boiling, freezing).
Ideal vs. non-ideal solutions.
Basic stoichiometry and mole concept.

Common mistakes

Forgetting to use the van't Hoff factor (i) for electrolytes.
Confusing molality (m) with molarity (M) in boiling point elevation and freezing point depression calculations.
Using solvent's molar mass instead of solute's (or vice-versa) when calculating mole fractions or molar masses.
Incorrectly identifying the solvent and solute in a problem.
Not converting units (e.g., temperature from Celsius to Kelvin for osmotic pressure).
Assuming all solutions are ideal.

Keywords

Colligative
Vapour Pressure
Boiling Point
Freezing Point
Osmotic Pressure
Raoult's Law
Ebullioscopic Constant
Cryoscopic Constant
Molality
Molarity
Van't Hoff factor
Dissociation
Association
Semi-permeable Membrane
Isotonic
Hypotonic
Hypertonic

Practice preview

Which of the following properties is a colligative property?…
easy
Which of the following 0.1 M aqueous solutions will have the highest freezing point?…
medium
Which of the following statements regarding colligative properties is INCORRECT?…
hard

Practice 9 questions