Raoult’s Law Explained for IB Chemistry

• Hexane + heptane
• Ethanol + propanol

These mixtures consist of molecules with similar sizes and intermolecular forces.

Vapor Pressure Lowering

Adding a non-volatile solute (one that does not evaporate) reduces vapor pressure.

Example:
Salt (NaCl) in water does not evaporate.
Therefore, water’s vapor pressure decreases according to Raoult’s Law.

Lower vapor pressure leads to:

• Higher boiling point
• Lower freezing point
• Changes in equilibrium behavior

This underpins many colligative property calculations.

Raoult’s Law for Volatile Solutes

If both components are volatile (e.g., two liquids), each contributes to total vapor pressure.

Then:

P(total) = X(A)P°(A) + X(B)P°(B)

This is essential for understanding:

• Fractional distillation
• Ideal mixtures
• Behavior of miscible liquids

The more volatile component contributes more to the vapor.

Positive and Negative Deviations (HL Concept)

Real solutions often deviate from Raoult’s Law.

Positive deviations

• The mixture has weaker intermolecular forces than expected
• Vapor pressure is higher than Raoult’s Law predicts
  Examples:
• Ethanol + hexane
• Acetone + carbon disulfide

Because molecules attract each other less strongly, they escape more easily.

Negative deviations

• The mixture has stronger intermolecular forces than expected
• Vapor pressure is lower
  Examples:
• Nitric acid + water
• Chloroform + acetone

Extra attraction makes vaporization harder.

These deviations are often tested in HL data-based questions.

Applications of Raoult’s Law

1. Boiling Point Elevation

Lower vapor pressure means a solution must reach a higher temperature before its vapor pressure equals external pressure.

2. Freezing Point Depression

Lower vapor pressure leads to a lower freezing point.

3. Fractional Distillation

Raoult’s Law helps predict how mixtures separate based on volatility differences.

4. Calculating vapor composition

When a mixture evaporates, the vapor is richer in the more volatile component.

Common IB Misunderstandings

• Thinking a solute increases vapor pressure (it decreases it).
• Forgetting that Raoult’s Law applies only to ideal solutions.
• Confusing mole fraction with percent composition.
• Assuming all deviations are positive—both types exist.

Carefully checking the nature of the solution avoids these errors.

FAQs

Does Raoult’s Law apply to all solutions?

No. It applies best to ideal solutions. Real solutions often show deviations due to differences in intermolecular forces.

Why does vapor pressure decrease when solute is added?

Because fewer solvent molecules reach the surface, reducing evaporation.

Can Raoult’s Law be used for solutions with volatile solutes?

Yes. Each volatile component contributes its own partial pressure based on its mole fraction.

Conclusion

Raoult’s Law describes how vapor pressure depends on the mole fraction of solvent in an ideal solution. Adding a solute lowers vapor pressure, influences boiling and freezing points, and determines how mixtures behave during distillation. Understanding this law provides essential insight into thermodynamics and solution chemistry in the IB curriculum.

Learn why the mole allows chemists to connect atomic-scale particles to measurable quantities used in real experiments.