Vapor Pressure Calculator
Solve Raoult's law P = X·P° for vapor pressure — either the lowering caused by a non-volatile dissolved solute, or the total vapor pressure of an ideal mixture of two volatile liquids. Two 3D diagrams compare a pure solvent to one with a dissolved solute, and charts show how vapor pressure changes with composition.
Reviewed by the ToolNestr Editorial Team — July 2026
Pure solvent vs. solution with solute
1. Pure solvent
Only solvent molecules (blue) at the surface — nothing dilutes their escape into vapor.
2. Solution with dissolved solute
Solute particles (amber) take up surface space, lowering the effective vapor pressure.
Vapor pressure graphs
How it works
The core idea in one line: a liquid's vapor pressure comes from how many of its own molecules are free to escape into the vapor phase, so anything that dilutes those molecules — whether an inert solute taking up space or a second volatile liquid sharing the surface — proportionally changes the vapor pressure.
P_solution = X_solvent · P°_solvent
Raoult's law — non-volatile solute case
ΔP = X_solute · P°_solvent
vapor pressure lowering caused by the solute
P_total = X_A·P°_A + X_B·P°_B
ideal mixture of two volatile liquids A and B
Every liquid has molecules constantly escaping into vapor and recondensing, and vapor pressure measures how much escaped vapor it takes to reach equilibrium with the liquid. Raoult's law says that a component's contribution to vapor pressure is simply its mole fraction times its own pure vapor pressure. Dissolve a non-volatile solute and it dilutes the solvent at the surface, lowering vapor pressure proportionally to how much solute is present. Mix two volatile liquids instead, and both contribute their own share to a combined total vapor pressure — the same underlying law, just applied twice.
Worked example 1 — glucose dissolved in water
Given: Water's pure vapor pressure at 25°C, P° = 23.8 mmHg. 5 mol of water is mixed with 0.5 mol of glucose (non-volatile).
The vapor pressure dropped by about 9%, matching exactly the 9.09% mole fraction of glucose — the lowering scales directly with how much of the solution is solute, regardless of what the solute actually is.
Worked example 2 — an ideal benzene-toluene mixture
Given: Benzene P° = 95.1 mmHg, toluene P° = 28.4 mmHg. Mixture is 60% benzene, 40% toluene by mole fraction.
Unlike the solute case, both components here are volatile and both contribute to the total vapor pressure — this is the general form of Raoult's law for a mixture of two liquids that can both evaporate.
Vapor pressure lowering vs. mole fraction of solute
Water at 25°C, P° = 23.8 mmHg — a non-volatile solute at increasing mole fraction.
| Solute mole fraction | Solution vapor pressure | Lowering (ΔP) |
|---|---|---|
| 0.05 | 22.61 mmHg | 1.19 mmHg |
| 0.091 ★ | 21.64 mmHg | 2.16 mmHg |
| 0.20 | 19.04 mmHg | 4.76 mmHg |
| 0.40 | 14.28 mmHg | 9.52 mmHg |
★ Reference row (worked example 1). Lowering scales linearly with solute mole fraction — double the solute fraction, double the vapor pressure drop.
Where vapor pressure actually matters
🧴 Perfume and fragrance formulation
Perfumers blend volatile compounds with different vapor pressures to control how a scent evolves over time — high-vapor-pressure "top notes" evaporate first, while lower-vapor-pressure "base notes" linger far longer.
⛽ Fuel blending
Gasoline formulations are engineered with specific vapor pressure targets — seasonal blends use lower vapor pressure in summer to reduce evaporative emissions, and higher vapor pressure in winter so engines start reliably in the cold.
🌡️ Distillation and separation processes
Raoult's law for volatile mixtures is the foundation of distillation calculations, letting chemical engineers predict vapor composition at each stage of separating a mixture like crude oil into its components.
🧪 Determining molar mass experimentally
Measuring vapor pressure lowering for a solution of known mass is one classic way to determine the molar mass of an unknown non-volatile solute, since the lowering depends directly on moles of solute present.
Common misconceptions
"Any dissolved substance lowers a solvent's vapor pressure the same way."
This is only true for a non-volatile solute. Dissolving a volatile substance instead contributes its own vapor pressure to the total, following the two-liquid form of Raoult's law — the total vapor pressure can even end up higher than either pure liquid's, depending on composition.
"Vapor pressure lowering depends on what the solute is chemically."
Like other colligative properties, vapor pressure lowering for a non-volatile solute depends only on the mole fraction of solute particles present, not on their chemical identity — equal mole fractions of different non-volatile solutes lower vapor pressure by the same amount.
"A solution's vapor pressure can never exceed either pure component's."
For an ideal mixture of two volatile liquids, the total vapor pressure is always between the two pure vapor pressures — but real, non-ideal mixtures can show positive deviations from Raoult's law, producing a total vapor pressure that exceeds even the more volatile pure liquid.
"Vapor pressure and boiling point are unrelated properties."
They are directly connected: boiling happens exactly when a liquid's vapor pressure rises to match the surrounding atmospheric pressure. Anything that lowers vapor pressure (like a dissolved solute) necessarily raises the temperature needed to reach that same atmospheric pressure — this is precisely why vapor pressure lowering and boiling point elevation are two sides of the same effect.
Formula sources & further reading
The formulas here are standard, traceable to:
- • OpenStax, Chemistry 2e — Chapter 11, "Solutions and Colloids" (free, peer-reviewed). openstax.org
- • Atkins & de Paula, Physical Chemistry — Ideal and real solutions chapters.
- • Brown, LeMay & Bursten, Chemistry: The Central Science — Chapter 13, Properties of Solutions.
P = X·P° (Raoult's law). Results are rounded for display.
How to use this calculator
Choose your mode
Non-volatile solute (colligative lowering), or ideal mixture of two volatile liquids.
Enter pure vapor pressures
Look these up for your solvent(s) at the relevant temperature.
Enter composition
Moles of solvent/solute, or mole fractions for a two-liquid mixture.
Related tools
Frequently asked questions
What is Raoult's law?
Raoult's law states that a solvent's vapor pressure above a solution equals its mole fraction times its pure vapor pressure: P = X·P°. For a mixture of two volatile liquids, the total vapor pressure is the sum of each component's contribution.
Why does dissolving a solute lower vapor pressure?
A non-volatile solute takes up some of the surface area where solvent molecules would otherwise escape into vapor, diluting the solvent's effective concentration — fewer solvent molecules at the surface means a lower vapor pressure at any given temperature.
What is an ideal solution?
An ideal solution is one where interactions between different molecules are essentially the same as interactions between identical molecules, so Raoult's law holds closely across the entire range of composition — real mixtures like benzene and toluene approximate this well.
How is vapor pressure lowering related to boiling point elevation?
They are two views of the same phenomenon: lowering a liquid's vapor pressure means it needs a higher temperature to reach the point where vapor pressure equals atmospheric pressure (boiling) — vapor pressure lowering is the underlying cause of boiling point elevation.
Do all solutions follow Raoult's law exactly?
No — Raoult's law describes ideal behavior. Real solutions with strong or weak interactions between different molecules (compared to like molecules) show positive or negative deviations from the law, especially at higher solute concentrations.