Raoult's Law Calculator — Vapour Pressure of a Solution

Background.

Warm an equal-mole mixture of benzene and toluene to 25 °C and the vapour above it is not half and half. It is about 77 percent benzene. That lopsided vapour is the reason distillation works, and Raoult's law predicts it from two facts about each liquid: how much of it is in the mixture, and how readily it evaporates on its own.

The law is short. Each volatile component sends vapour into the space above the liquid in proportion to its share of the molecules, so its partial pressure is its mole fraction times the vapour pressure it would have if it were pure: p_A = x_A × p*_A. Dissolve something that does not evaporate at all, such as sugar, salt or a polymer, and the solvent's vapour pressure falls by exactly the solute's share of the molecules. That fall is what pushes the boiling point of a syrup above 100 °C and the freezing point of brine below zero.

There is no temperature box. Every pressure in Raoult's law belongs to one temperature, and that temperature enters only through the pure-component vapour pressures you type in, so both must be looked up for the same conditions. A benzene value at 25 °C paired with a toluene value at 80 °C gives arithmetic that means nothing.

The model is an ideal solution, one in which unlike molecules attract each other about as strongly as like ones do. Benzene and toluene, two similar non-polar aromatics, come close. Ethanol and water do not: hydrogen bonding pushes their real vapour pressure above the Raoult line, and the pair forms an azeotrope near 95.6 percent ethanol by mass that no simple still can get past.

What is raoult's law calculator (vapour pressure of solutions)?

Raoult's law states that in an ideal liquid solution the partial vapour pressure of each component equals its liquid-phase mole fraction multiplied by the saturation vapour pressure of the pure component at the same temperature. François-Marie Raoult established it experimentally in the 1880s. Strictly it is a limiting law, exact as a component approaches purity (x → 1). IUPAC writes the rigorous form with the solvent's activity in place of its mole fraction, and defines an activity coefficient 'referenced to Raoult's law', f = a/x, to measure how far a real mixture strays. Every result here assumes f = 1. The opposite limit, a component present only in traces, is governed by Henry's law instead.

How to use this calculator.

1. 01Under 'What kind of solution is it?', choose two volatile components when both liquids evaporate appreciably at your temperature, or a non-volatile option when the dissolved substance is a sugar, salt, polymer or anything else that barely evaporates.
2. 02Fix the working temperature first, then look up both pure-component vapour pressures at that temperature. The NIST Chemistry WebBook lists Antoine constants for most solvents in the form log₁₀(p/bar) = A − B/(T + C), with T in kelvin; multiply the result in bar by 100 to get kPa.
3. 03Enter the composition of the liquid, not the vapour, as the mole fraction of A between 0 and 1, so half is 0.5. With the mole-amounts option, enter moles of solvent A and moles of solute B instead: grams of water divided by 18.015 gives moles of water.
4. 04For an ionic solute, enter moles of dissolved particles rather than moles of salt: roughly twice the formula amount for sodium chloride and three times for calcium chloride.
5. 05Read the total vapour pressure first. In the two-liquid option, set the vapour mole fraction y_A beside the x_A you entered to see how much one evaporation step enriches the vapour; in the non-volatile options, check that the relative lowering equals the solute's mole fraction.

The formula.

For every component that evaporates, p_i = x_i × p*_i. With two volatile liquids x_B = 1 − x_A, so the total pressure P = x_A p*_A + (1 − x_A) p*_B is a straight line running from p*_B at x_A = 0 to p*_A at x_A = 1. Dalton's law gives the vapour composition, y_A = p_A / P. Rearranged, y_A / x_A = p*_A / P, and since P always lies between the two pure values, the vapour is richer than the liquid in whichever component has the higher pure vapour pressure. The two compositions coincide only when p*_A = p*_B, and then no distillation can separate the pair.

With a non-volatile solute, p_B is zero and P = p_A. The lowering is Δp = p*_A − p_A = (1 − x_A) p*_A, and dividing by p*_A leaves Δp / p*_A = 1 − x_A: the fractional drop equals the solute's mole fraction, whatever the solute is. That indifference to chemical identity makes the lowering a colligative property, the root of boiling-point elevation, freezing-point depression and osmotic pressure.

The mole fraction must be above 0 and at most 1 (exactly 1 is pure A), and both pure vapour pressures must be positive. Because Δp is measured against A's own partial pressure, it can never come out negative.

A worked example.

Frequently asked questions.

References& sources.

1. [1]International Union of Pure and Applied Chemistry. Compendium of Chemical Terminology (the 'Gold Book'), 5th edition, online version 5.0.0 (2025), entry 'Henry's law' (H02783), which also defines Raoult's law: 'For the solvent (A) the relationship is called Raoult's law, and the proportionality factor is the fugacity of the pure solvent, p̃*_A: p_A = p̃*_A a_A.' Source document: Pure Appl. Chem. 1984, 56, 567 (Physicochemical quantities and units in clinical chemistry, Recommendations 1983), p. 571. Independent standards body; open access; retrieved and quoted 2026-07-29. The alias R05135 'Raoult's law' resolves to this entry.
2. [2]International Union of Pure and Applied Chemistry. Quantities, Units and Symbols in Physical Chemistry (the 'Green Book'), 3rd edition, 2nd printing 2012, p. 59: 'activity coefficient referenced to Raoult's law f, f_B = a_B / x_B, SI unit 1', with the note that the quantity 'applies to pure phases, substances in mixtures, or solvents'. This is the formal definition of the departure from the ideal-solution assumption this calculator makes (f = 1). Consulted as a SECOND authority independent of the Gold Book entry above; agrees. Open access PDF, text extracted and read locally; retrieved 2026-07-29.
3. [3]National Institute of Standards and Technology. NIST Chemistry WebBook, SRD 69 — benzene (CAS 71-43-2), Antoine equation parameters, log₁₀(p/bar) = A − B/(T + C) with T in K. Willingham, Taylor, Pignocco & Rossini (1945): A = 4.01814, B = 1203.835, C = −53.226, valid 287.70–354.07 K, giving 12.677 kPa (95.08 Torr) at 298.15 K — the p*_A used in the worked example. Note the page also lists an independent short-range fit (Deshpande & Pandya 1967, 297.9–318.0 K) that gives 12.16 kPa at the same temperature, 4.1 % lower; this page uses the Willingham fit. Open access; retrieved 2026-07-29.
4. [4]National Institute of Standards and Technology. NIST Chemistry WebBook, SRD 69 — toluene (CAS 108-88-3), Antoine equation parameters. Pitzer & Scott (1943): A = 4.14157, B = 1377.578, C = −50.507, valid 273–323 K, giving 3.7915 kPa (28.44 Torr) at 298.15 K — the p*_B used in the worked example. Cross-checked against the independent fit of Besley & Bottomley (1974), A = 4.23679, B = 1426.448, C = −45.957, valid 273.13–297.89 K, which gives 3.80727 kPa — agreement to 0.42 %. Open access; retrieved 2026-07-29.
5. [5]National Institute of Standards and Technology. NIST Chemistry WebBook, SRD 69 — Thermophysical Properties of Fluid Systems, water saturation table, computed from the IAPWS Formulation 1995 (Wagner & Pruss, J. Phys. Chem. Ref. Data 31, 387, 2002). Saturation pressure of water at 25.0000 °C = 3.169 93 kPa, the value quoted in the vapour-pressure-lowering FAQ and used as a reference point in the input hints. Open access; value retrieved directly from the data endpoint 2026-07-29.