This Raoult’s law calculator finds a solution’s vapor pressure from the solvent’s mole fraction and the pure solvent’s vapor pressure — the colligative lowering behind boiling, freezing and humidity behavior.

Solution Vapor Pressure P–
Lowering ΔP–
χ solvent used–

ΔP = χ_solute × P° — the lowering depends only on the solute’s mole fraction, not its identity (for non-volatile, non-electrolyte solutes in ideal solutions). Electrolytes multiply their particle count (van’t Hoff i): for NaCl, effective χ_solute ≈ 2 × formula-unit fraction. Volatile solutes break the simple form — each component then contributes its own Raoult term (the distillation equations).

P = χ_solvent × P° — The Surface Is Diluted

The intuition is geometric: evaporation happens at the surface, and solute occupies a χ_solute share of it — so escaping tendency falls by that share. Mole fractions come from our Mole Fraction Calculator. The same dilution logic drives freezing-point depression and osmotic pressure: one mole fraction, four colligative consequences. Part of our Chemistry Calculators collection.

Worked Examples (Verified)

  • χ = 0.90 over water at 25 °C (P° 23.8 torr) (defaults): P = 21.42 torr, lowering 2.38 torr (10%) — relative humidity over a solution is this number wearing a meteorologist’s hat
  • From moles: 1 mol glucose in 9 mol water → χ 0.90, same result — routes converge, as mole-fraction bookkeeping promises
  • Molar mass from vapor pressure (historical Dumas-adjacent method): a measured ΔP with known masses gives χ_solute, hence moles, hence M — colligative properties were the 19th century’s molecular scales
  • Seawater check: χ_water ≈ 0.98 → vapor pressure ≈ 2% below pure — small, yet enough to matter for evaporation and desalination energy math

Deviations Are Data

  • Positive deviations (P above Raoult): unlike solvent–solute attractions (ethanol–water), up to azeotropes — minimum-boiling mixtures distillation cannot cross.
  • Negative deviations (P below): strong cross-attraction (acetone–chloroform H-bonding), maximum-boiling azeotropes.
  • Raoult is the ideal-solution law; Henry’s law governs the solute’s own (volatile) behavior in the dilute limit — real mixtures quote both.
  • For electrolytes, replace χ_solute with i × χ (particles again) — dilute NaCl lowers roughly twice the naive prediction.

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Frequently Asked Questions

What is Raoult’s law?

P_solution = χ_solvent × P°_solvent: a non-volatile solute lowers vapor pressure in proportion to the solvent’s mole fraction. Equivalently ΔP = χ_solute × P°. Water (23.8 torr at 25 °C) with χ 0.90 gives 21.42 torr.

Why does the solute’s identity not matter?

Because (in the ideal limit) the mechanism is surface dilution and entropy of mixing — counting effects. Any dissolved non-volatile particle blocks the same share of surface. Deviations from identity-independence measure how non-ideal the interactions are, which is itself useful information.

What if the solute is volatile too?

Then each component contributes Pᵢ = χᵢ × P°ᵢ and total pressure is their sum (Raoult applied per-component) — the foundation of distillation theory, where the vapor is richer in the more volatile component. This calculator handles the single-solvent/non-volatile-solute case.

How does Raoult’s law connect to boiling and freezing?

Lower vapor pressure means the liquid must be heated further for its vapor pressure to reach atmospheric pressure (boiling elevation), and the chemical-potential shift simultaneously depresses freezing. One cause (diluted solvent activity), both effects — plus osmotic pressure — the colligative quartet.

Do electrolytes follow Raoult’s law?

With the van’t Hoff factor multiplying the solute mole fraction: NaCl counts ≈ 2 particles per formula unit, CaCl₂ ≈ 3 (dilute ideals; real values sag with concentration). Enter the effective χ (solvent fraction computed with particle counts) for electrolyte estimates.

Is this Raoult’s law calculator free?

Yes — free, instant, no sign-up.