ChemistrySolutionsClass 12

Raoult's law & colligative properties: interactive simulation

Colligative properties depend solely on the number of solute particles relative to solvent molecules, not on their chemical identity. Adding a non-volatile solute lowers solvent vapor pressure, which elevates boiling point, depresses freezing point and creates osmotic pressure.

Useful for: JEE Main, JEE Advanced, NEET, CBSE Boards · 3 modes in one lab

What you can explore

  1. 1

    Raoult's Law & Azeotropes

    P-x binary vapor pressure diagram with positive and negative deviations.
  2. 2

    4 Colligative Properties

    Live calculator comparing boiling point, freezing point, osmotic pressure and RLVP.
  3. 3

    Van 't Hoff Factor & Dissociation

    Degree of dissociation α and association dimerization simulator.

Try this

Plot binary mixture vapor pressure curves to see ideal behavior, positive deviations (minimum boiling azeotropes) and negative deviations (maximum boiling azeotropes), and see how the Van ’t Hoff factor i accounts for ionic dissociation and molecular association.

  1. 1

    In Colligative sim, compare 1 molal Glucose (i=1) vs 1 molal NaCl (i=2) vs 1 molal CaCl₂ (i=3).

    What you should see

    Boiling point elevation and freezing point depression triple for CaCl₂ compared to glucose because it produces 3 particles in solution!

  2. 2

    In Raoult sim, switch from Ideal to +ve Deviation.

    What you should see

    Total vapor pressure rises above Raoult’s ideal line (e.g. ethanol-acetone mixture, ΔH_mix > 0).

  3. 3

    In Van ’t Hoff sim, switch to Association.

    What you should see

    For dimerization of benzoic acid in benzene, i drops below 1 (i = 0.5 for 100% dimerization).

Formulas behind it

Elevation in boiling point

ΔTb=i⋅Kb⋅m,Kb=R⋅M1⋅Tb21000⋅ΔHvap\Delta T_b = i \cdot K_b \cdot m, \quad K_b = \frac{R \cdot M_1 \cdot T_b^2}{1000 \cdot \Delta H_{\text{vap}}}

Depression in freezing point

ΔTf=i⋅Kf⋅m,Kf=R⋅M1⋅Tf21000⋅ΔHfus\Delta T_f = i \cdot K_f \cdot m, \quad K_f = \frac{R \cdot M_1 \cdot T_f^2}{1000 \cdot \Delta H_{\text{fus}}}

Osmotic pressure formula

π=i⋅C⋅R⋅T=i(n2V)RT\pi = i \cdot C \cdot R \cdot T = i \left(\frac{n_2}{V}\right) R T

Van ’t Hoff factor for dissociation

i=1+(n−1)α  ⟺  α=i−1n−1i = 1 + (n - 1) \alpha \iff \alpha = \frac{i - 1}{n - 1}

Common mistakes

  • Forgetting the Van ’t Hoff factor i in calculations involving electrolytes like NaCl, BaCl₂ or Al₂(SO₄)₃.
  • Confusing molality (moles solute per kg solvent) with molarity (moles solute per L solution) — molality is independent of temperature.
  • Assuming reverse osmosis occurs spontaneously — reverse osmosis requires applying an external hydrostatic pressure GREATER than the osmotic pressure π.
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FAQ

Why is osmotic pressure the preferred method for measuring molar mass of polymers and biomolecules?

Because osmotic pressure produces measurable values at room temperature even for extremely dilute solutions, and high temperatures that could denature proteins are avoided.

What is an azeotropic mixture?

A constant-boiling liquid mixture that distills without changing its composition, boiling at a temperature lower (minimum boiling) or higher (maximum boiling) than its components.

What is an isotonic solution?

Two solutions that have identical osmotic pressure at the same temperature (e.g. 0.9% w/v NaCl is isotonic with human red blood cells).

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