Raoult's Law Calculator

Calculate the vapor pressure of an ideal solution using Raoult's Law. Enter the pure solvent vapor pressure and mole fraction to find the solution's vapor pressure.

Vapor pressure of pure solvent at given temperature

Mole fraction of solvent in solution

Temperature of the solution

Moles of non-volatile solute

Moles of solvent

Raoult's Law Equation: P_solution = χ₁ × P°₁ Where: • P_solution = Vapor pressure of solution • χ₁ = Mole fraction of solvent • P°₁ = Vapor pressure of pure solvent Mole Fraction Calculation: χ₁ = moles solvent / (moles solvent + moles solute) χ₂ = moles solute / (moles solvent + moles solute) Pressure Lowering: ΔP = P°₁ - P_solution = P°₁ × (1 - χ₁) Relative Pressure Lowering: ΔP / P°₁ = 1 - χ₁ = χ₂
Example 1 (Sugar in Water): Pure water vapor pressure at 25°C = 23.8 mmHg Mole fraction of water = 0.90 P_solution = 0.90 × 23.8 = 21.42 mmHg Pressure lowering = 2.38 mmHg (10%) Example 2 (NaCl Solution): Moles water = 5.0 mol, Moles NaCl = 0.5 mol χ₁ = 5.0 / 5.5 = 0.909 Pure water P° = 23.8 mmHg P_solution = 0.909 × 23.8 = 21.63 mmHg Example 3 (Benzene-Toluene - Ideal): Benzene χ = 0.5, P° benzene = 95 mmHg Toluene χ = 0.5, P° toluene = 28 mmHg P_total = 0.5×95 + 0.5×28 = 61.5 mmHg Example 4 (Calculate from moles): Glucose solution: 1 mol glucose, 10 mol water χ_water = 10/11 = 0.909 If P° = 31.8 mmHg (at 35°C) P_solution = 0.909 × 31.8 = 28.9 mmHg

What is Raoult's Law?

Raoult's Law states that the vapor pressure of a solvent above a solution equals the vapor pressure of the pure solvent multiplied by its mole fraction in the solution: P_solution = χ₁ × P°₁. It assumes ideal solution behavior where solute molecules do not interact differently than solvent molecules.

When does Raoult's Law apply?

Raoult's Law works best for dilute solutions where the solute is non-volatile and the solution behaves ideally. Deviations occur at high concentrations due to solute-solvent interactions. Positive deviations happen when solute-solvent attractions are weaker; negative when attractions are stronger.

What is the difference between ideal and non-ideal solutions?

Ideal solutions follow Raoult's Law perfectly across all concentrations (like benzene-toluene). Non-ideal solutions show deviations: positive (higher vapor pressure, like acetone-water) or negative (lower vapor pressure, like chloroform-acetone). The activity coefficient (γ) quantifies these deviations.

How does mole fraction affect vapor pressure?

As the mole fraction of solvent decreases (more solute), vapor pressure decreases proportionally. A solution with 80% solvent (χ₁ = 0.8) will have 80% of pure solvent vapor pressure. This is the basis for boiling point elevation and freezing point depression in solutions.

What is the relationship between Raoult's Law and boiling point?

Raoult's Law explains boiling point elevation: adding a non-volatile solute lowers vapor pressure, so a higher temperature is needed to reach atmospheric pressure. The boiling point elevation ΔTb = Kb × m, where Kb is the ebullioscopic constant and m is molality.

🧮
Partial Pressure Calculator
🧮
Mole Fraction Calculator
🧮
Freezing Point Depression Calculator
🧮
Boiling Point Elevation Calculator