Vapor Pressure Calculator (Raoult's Law)
Calculate total and partial vapor pressures in ideal liquid mixtures using Raoult's Law.
Find vapor composition above the liquid.
Includes benzene-toluene example.
Raoult’s Law describes the vapor pressure of ideal solutions — mixtures where components have similar intermolecular forces.
Partial pressure of component A:
P_A = x_A × P°_A
Total vapor pressure:
P_total = x_A × P°_A + x_B × P°_B
Where:
- x_A, x_B = mole fractions in the liquid phase (x_A + x_B = 1)
- P°_A, P°_B = vapor pressures of pure components
Vapor composition (mole fraction in vapor):
y_A = P_A / P_total = x_A × P°_A / P_total
Positive and negative deviations from Raoult’s Law:
- Ideal (Raoult’s Law): A-B interactions ≈ A-A and B-B interactions (e.g., benzene-toluene)
- Positive deviation: A-B interactions weaker than pure components (e.g., ethanol-water, acetone-hexane) → P_total > Raoult’s Law
- Negative deviation: A-B interactions stronger (e.g., acetone-chloroform, HCl-water) → P_total < Raoult’s Law
Benzene-toluene system at 80°C (approximately ideal):
- P°_benzene ≈ 760 mmHg (normal boiling point!)
- P°_toluene ≈ 290 mmHg
Applications:
- Fractional distillation (separating components by boiling point)
- Understanding azeotropes (constant-boiling mixtures that cannot be separated by simple distillation)
- Colligative properties (related: vapor pressure lowering by non-volatile solutes)
Colligative properties connection: When a non-volatile solute dissolves, x_solvent decreases → vapor pressure decreases. ΔP = x_solute × P°_solvent (Raoult’s Law for non-volatile solute — see vapor pressure lowering calculator).