Why is molality preferred over molarity for studying colligative properties?

Molarity depends on volume of solution, which changes with temperature (liquids expand/contract). Molality depends on mass of solvent, which is temperature-independent. Since colligative properties (boiling point elevation, freezing point depression) depend on composition at a given temperature, molality gives more accurate and consistent results.

What are colligative properties and why don't they depend on solute nature?

Colligative properties depend ONLY on the NUMBER of solute particles (not their nature/identity): vapour pressure lowering, boiling point elevation (ΔTb = Kb·m), freezing point depression (ΔTf = Kf·m), osmotic pressure (π = MRT). They don't depend on solute identity because they result from diluting the solvent (reducing its mole fraction), regardless of what is dissolved.

What is van't Hoff factor and when is it greater than 1?

Van't Hoff factor i = actual moles of particles in solution / moles of solute dissolved. For non-electrolytes (sugar, urea): i = 1. For electrolytes that dissociate: i > 1 (NaCl: i=2; BaCl₂: i=3; Al₂(SO₄)₃: i=5 theoretically). For association (e.g., acetic acid in benzene forms dimers): i < 1.

Concentration Terms, Molarity and Colligative Properties

Solutions • Class 12 Chemistry • NCERT • CBSE

Molarity M = moles of solute/volume of solution (L). Molality m = moles of solute/kg of solvent. Mole fraction x = moles of component/total moles. Raoult's Law: P = x_solvent × P°. Colligative properties: vapour pressure lowering, boiling point elevation (ΔTb = Kb·m), freezing point depression (ΔTf = Kf·m).

Key Formulas

M = moles/volume(L)
m = moles/kg(solvent)
ΔTb = Kb × m
ΔTf = Kf × m
π = MRT (osmotic pressure)
P = K_H × x (Henry's Law)

Frequently Asked Questions

Why is molality preferred over molarity for studying colligative properties?: Molarity depends on volume of solution, which changes with temperature (liquids expand/contract). Molality depends on mass of solvent, which is temperature-independent. Since colligative properties (boiling point elevation, freezing point depression) depend on composition at a given temperature, molality gives more accurate and consistent results.
What are colligative properties and why don't they depend on solute nature?: Colligative properties depend ONLY on the NUMBER of solute particles (not their nature/identity): vapour pressure lowering, boiling point elevation (ΔTb = Kb·m), freezing point depression (ΔTf = Kf·m), osmotic pressure (π = MRT). They don't depend on solute identity because they result from diluting the solvent (reducing its mole fraction), regardless of what is dissolved.
What is van't Hoff factor and when is it greater than 1?: Van't Hoff factor i = actual moles of particles in solution / moles of solute dissolved. For non-electrolytes (sugar, urea): i = 1. For electrolytes that dissociate: i > 1 (NaCl: i=2; BaCl₂: i=3; Al₂(SO₄)₃: i=5 theoretically). For association (e.g., acetic acid in benzene forms dimers): i < 1.

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