Raoult's Law

Vapor pressure of an ideal solution equals mole fraction times pure component pressure

Raoult's Law Formula

Psolution = Xsolvent * P°solvent
For each volatile component i: P_i = X_i * P°_i

Variables

  • Psolution = vapor pressure of solution
  • Xsolvent = mole fraction of solvent
  • P°solvent = vapor pressure of pure solvent

Assumptions

  • Ideal solution behavior
  • Volatile solvent (or component) obeys Raoult's law
  • No strong solute-solvent interactions beyond ideality

Step-by-Step Example

Problem:

What is the vapor pressure of a solution made by dissolving 20.0 g NaCl in 180.0 g water at 25 C? P°water = 23.8 mmHg.

1) Moles

NaCl moles = 20.0 g / 58.44 = 0.342 mol; water moles = 180.0 g / 18.02 = 9.99 mol.

2) Mole fraction of water

Xwater = 9.99 / (9.99 + 0.342) = 0.967

3) Vapor pressure

Psolution = 0.967 * 23.8 mmHg = 23.0 mmHg

Answer:

The solution vapor pressure is 23.0 mmHg (ideal assumption).

Common Mistakes to Avoid

Using mass fraction

Always use mole fractions, not mass fractions.

Nonvolatile solutes

Only volatile components contribute to vapor pressure.

Non-ideal solutions

Strong interactions cause deviations; activity coefficients needed.

Temperature mismatch

Use P° at the same temperature as the solution.

Related Calculators

Mole Fraction

X = moles component / total

Osmotic Pressure

Π = iMRT

Freezing Point Depression

ΔTf = i Kf m

Vapor Pressure Formula

ln(P₂/P₁) = -ΔHvap/R(1/T₂-1/T₁)

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Frequently Asked Questions

When does Raoult's law fail?

For strong solute-solvent interactions, electrolytes at higher concentration, or hydrogen bonding systems.

Does salt lower vapor pressure?

Yes, nonvolatile solutes lower vapor pressure proportionally to solvent mole fraction.

What about multiple volatile components?

Apply P_i = X_i P°_i for each component and sum to total pressure.