Raoult's Law & Vapor Pressure Solver
Solve Raoult's Law for a nonvolatile solute's vapor pressure lowering, or find the total and partial vapor pressures of an ideal binary solution of two volatile liquids, check for positive or negative deviation, and see the vapor-phase composition — with full step-by-step working.
Pick a mode, then enter the values you already know.
Result
Solution vapor pressure
Lowered by 2.1641 mmHg from the pure solvent
0.90907
Mole fraction (solvent)
0.09093
Mole fraction (solute)
4.99445 mol
Moles solvent
0.49956 mol
Moles solute
Step-by-Step Raoult's Law Calculation
Here's exactly how this answer was calculated, one step at a time.
Step 1: Convert mass to moles for both components
Moles of solvent = 90 g ÷ 18.02 g/mol = 4.99445 mol; Moles of solute = 171 g ÷ 342.3 g/mol = 0.49956 molStep 2: Find the mole fraction of the solvent
Only the solvent's mole fraction matters here because the solute is nonvolatile — it doesn't contribute any vapor pressure of its own.
X(solvent) = 4.99445 ÷ (4.99445 + 0.49956) = 0.90907Step 3: Apply Raoult's Law: P(solution) = X(solvent) × P°(solvent)
P(solution) = 0.90907 × 23.8 mmHg = 21.6359 mmHgStep 4: Find the vapor pressure lowering
ΔP = P°(solvent) − P(solution) = X(solute) × P°(solvent) = 0.09093 × 23.8 mmHg = 2.1641 mmHgStep 5: Result
Solution vapor pressure ≈ 21.6359 mmHg, lowered by 2.1641 mmHg from the pure solvent.
Raoult's Law & Vapor Pressure Solver: Two Classic Problems, One Tool
This free calculator solves the two most common problems built around Raoult's Law. The first mode handles a nonvolatile solute dissolved in a solvent — think sugar or salt in water — and finds exactly how much the solution's vapor pressure drops compared to the pure solvent. The second mode handles a solution of two volatile liquids, where both components evaporate and contribute their own partial pressure, and finds the total vapor pressure, each partial pressure, and the composition of the vapor sitting above the liquid.
Every result comes with full step-by-step working, so whether you're a general chemistry student checking a textbook problem, a physical chemistry student exploring ideal versus non-ideal solutions, or a distillation or process engineer sketching out a quick vapor-liquid equilibrium estimate, you can see exactly how each number was produced.
What Is Raoult's Law?
Raoult's Law, named after French chemist François-Marie Raoult, describes how the vapor pressure of a liquid component changes once it's mixed into a solution. In its simplest form: P = X × P°, where P is the vapor pressure that component contributes within the solution, X is its mole fraction in the liquid, and P° is its vapor pressure as a pure substance at the same temperature.
The intuition behind it is straightforward: a liquid's vapor pressure comes from molecules at the surface escaping into the gas phase. Once you dilute that liquid with something else, fewer of its own molecules are sitting at the surface at any given moment, so fewer of them escape, and the vapor pressure drops in direct proportion to how much of the mixture is actually that component.
Nonvolatile Solutes and Vapor Pressure Lowering
When the dissolved solute is nonvolatile — meaning it essentially doesn't evaporate at all, like table sugar or table salt in water — only the solvent contributes any vapor pressure to the mixture. Since the solvent's mole fraction is always less than 1 once something else is dissolved in it, the solution's vapor pressure is always lower than the pure solvent's vapor pressure.
The amount of that drop is called vapor pressure lowering, and it's calculated as ΔP = X(solute) × P°(solvent). Because it depends only on how many solute particles are dissolved — not on what the solute actually is — vapor pressure lowering is one of the four classic colligative properties, alongside boiling point elevation, freezing point depression, and osmotic pressure. In fact, all four of these properties are mathematically connected: whatever mole fraction of solute you use for vapor pressure lowering is the same number driving the shift in boiling point and freezing point.
Two Volatile Liquids: Ideal Binary Solutions
When both components of a mixture are volatile — like a mixture of two organic solvents, or ethanol and water — each one follows Raoult's Law on its own. Component A contributes a partial pressure of X(A) × P°(A), and component B contributes X(B) × P°(B). Because both are evaporating into the same shared headspace, the total vapor pressure above the solution is simply the sum of the two partial pressures — this last step is really just Dalton's Law of partial pressures layered on top of Raoult's Law.
This calculator's second mode does exactly that: given the pure vapor pressure of each liquid and either the mole fraction directly or a mass and molar mass for each component, it works out both partial pressures, adds them for the total, and then divides each partial pressure by that total to find the vapor's own composition — a mole fraction the vapor phase, often written as y(A) and y(B) to distinguish it from the liquid-phase mole fractions X(A) and X(B).
Why the Vapor Is Richer in the More Volatile Component
One of the most useful consequences of Raoult's Law is that the vapor sitting above a boiling mixture is never the same composition as the liquid it came from — it's always enriched in whichever component has the higher pure vapor pressure (in other words, the lower boiling point). This single fact is the entire basis of distillation: repeatedly boiling off and re-condensing vapor gradually separates a mixture into its more-volatile and less-volatile parts, which is exactly how everything from crude oil refining to whiskey production and laboratory solvent purification works.
This calculator's vapor mole fraction output (y(A) and y(B)) is precisely the number a chemical engineer would use as the starting point for sketching a single equilibrium stage on a distillation McCabe-Thiele diagram, or simply for predicting which component will condense first when a two-liquid mixture is heated.
Ideal Solutions and Deviations from Raoult's Law
Raoult's Law assumes an ideal solution, where molecules of A and molecules of B attract each other with roughly the same strength that they attract their own kind. Mixtures of very similar liquids — like benzene and toluene, or hexane and heptane — come close enough to this assumption that Raoult's Law predicts their vapor pressure almost exactly across the whole range of compositions.
Most real mixtures deviate at least a little. A positive deviation happens when A and B molecules attract each other more weakly than they attract their own kind, so more molecules escape into the vapor phase than Raoult's Law expects — ethanol and hexane is a classic example. A negative deviation happens when A and B attract each other more strongly, often because they form hydrogen bonds with each other that neither pure liquid could form on its own — acetone and chloroform is the textbook example, since chloroform's hydrogen can hydrogen-bond with acetone's oxygen. This calculator lets you enter a measured total vapor pressure alongside the Raoult's Law prediction, and it will tell you whether your data shows a positive deviation, a negative deviation, or essentially ideal behavior.
How This Connects to Boiling Point and Distillation
A liquid boils at the temperature where its vapor pressure equals the surrounding atmospheric pressure. Since dissolving a nonvolatile solute always lowers a solvent's vapor pressure, that solution has to be heated to a higher temperature before its vapor pressure climbs back up to atmospheric pressure — this is exactly why boiling point elevation happens, and it's driven by the same mole fraction this calculator computes.
For a two-volatile-liquid mixture, the boiling point sits wherever the sum of the two partial pressures (the total pressure this calculator finds) equals atmospheric pressure. As you heat a mixture, the liquid composition constantly shifts as the more volatile component boils away faster, which is why a distillation curve tracks a changing boiling point rather than one fixed value.
Where This Formula Is Actually Used
General and physical chemistry courses use Raoult's Law to introduce colligative properties and to build the theoretical foundation for phase diagrams and vapor-liquid equilibrium. Chemical and process engineers use it as the simplest starting model for flash calculations, distillation column design, and solvent recovery systems, before moving on to more advanced models (like activity-coefficient models) that account for non-ideal behavior.
Everyday applications include understanding why saltwater takes longer to boil dry than fresh water, why antifreeze mixtures behave the way they do in a car radiator, why a bottle of two different solvents smells like a blend of both rather than just the more volatile one, and why fractional distillation can separate ethanol from water, or crude oil into gasoline, diesel, and other fractions.
Common Mistakes When Using Raoult's Law
The most frequent mistake is using the mole fraction of the solute instead of the solvent when finding a solution's vapor pressure directly — remember, P(solution) = X(solvent) × P°(solvent), not X(solute) × P°(solvent). The solute's mole fraction is what you multiply by P° to get the lowering, ΔP, not the solution's remaining vapor pressure.
Another common error in two-volatile-liquid problems is forgetting that the vapor-phase composition (y) is different from the liquid-phase composition (X) — students sometimes assume the vapor above a 50/50 mixture is also 50/50, when in reality it's shifted toward whichever component has the higher pure vapor pressure. It's also easy to forget that Raoult's Law is only exact for an ideal solution; for real, especially non-similar liquid pairs, the true vapor pressure can be noticeably higher or lower than a Raoult's Law calculation predicts, which is exactly what the deviation check in this calculator's advanced options is for.
Raoult's Law Calculator: Quick Reference and Disclaimer
Quick formulas: Nonvolatile solute — P(solution) = X(solvent) × P°(solvent); vapor pressure lowering ΔP = X(solute) × P°(solvent). Two volatile liquids — P(A) = X(A) × P°(A), P(B) = X(B) × P°(B), P(total) = P(A) + P(B), vapor mole fraction y(A) = P(A) ÷ P(total).
This calculator is intended for educational and general reference use, including general and physical chemistry coursework, and preliminary engineering estimates for distillation or vapor-liquid equilibrium problems. It assumes ideal-solution behavior and does not replace a proper activity-coefficient model, experimental vapor pressure data, or professional process engineering software for real industrial design work.
Frequently Asked Questions
What is Raoult's Law?
Raoult's Law states that the vapor pressure a solvent (or any volatile liquid) contributes to a solution equals its mole fraction in that solution multiplied by its vapor pressure as a pure substance: P = X × P°. It's one of the foundational ideas in solution chemistry and is the basis for all four colligative properties.
What is the formula for vapor pressure lowering?
Vapor pressure lowering is ΔP = X(solute) × P°(solvent), where X(solute) is the mole fraction of the dissolved, nonvolatile solute and P°(solvent) is the vapor pressure of the pure solvent at that temperature. This is mathematically the same as P°(solvent) minus the solution's actual vapor pressure.
Does Raoult's Law only work for nonvolatile solutes?
No. Raoult's Law applies to any component of an ideal solution, volatile or not. For a nonvolatile solute, only the solvent contributes vapor pressure, so the solution's total vapor pressure is simply X(solvent) × P°(solvent). For a mixture of two volatile liquids, each one follows Raoult's Law independently, and the two partial pressures add together (via Dalton's Law) to give the total vapor pressure.
What is an ideal solution?
An ideal solution is one where the attractive forces between different molecules (A-B interactions) are essentially the same strength as the attractive forces between identical molecules (A-A and B-B interactions). Under that condition, mixing doesn't release or absorb extra energy, and the solution follows Raoult's Law exactly across the whole composition range. Real solutions only approximate this when the two components are chemically very similar, like benzene and toluene.
What does positive or negative deviation from Raoult's Law mean?
A positive deviation happens when a solution's actual measured vapor pressure is higher than Raoult's Law predicts, which means the two different molecules attract each other more weakly than they attract their own kind — they'd almost rather escape into the vapor phase. A negative deviation is the opposite: the measured vapor pressure is lower than predicted, because the two different molecules attract each other more strongly than expected, often through hydrogen bonding, which holds more of them in the liquid phase.
How do I find the vapor-phase composition above a solution?
Once you know each component's partial pressure from Raoult's Law, divide that partial pressure by the total vapor pressure to get its mole fraction in the vapor: y(A) = P(A) ÷ P(total). This calculator does that step automatically in the two-volatile-liquids mode, and it's the same relationship that underlies distillation — the vapor above a boiling mixture is always richer in the more volatile component than the liquid it came from.
Can this calculator handle more than two components?
This tool is built specifically for the two most common Raoult's Law problems: a single nonvolatile solute in a solvent, and a two-component volatile mixture. For mixtures with three or more volatile components, or when you need mole fractions from several components at once, our Mole Fraction Calculator can work out each component's contribution before you apply Raoult's Law by hand.