Raoult's law vapor pressure calculator
Water at 23.8 torr with a solvent mole fraction of 0.85 gives a solution vapor pressure of 20.23 torr, a drop of 3.57 torr caused by the dissolved solute. Enter the mole fraction and the pure-solvent value. Free in your browser.
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Worked example
- Mole fraction
- 0.85
- Pure vapor pressure
- 23.8
- Vapor pressure
- 20.23
- Solute mole fraction
- 0.15
- Vapor pressure lowering
- 3.57
Raoult's law links the vapor pressure of a solvent in an ideal solution to how much of that solvent is present on a mole basis. For the solvent component the classical statement is P equals x solvent times P degree solvent, where x solvent is the mole fraction of solvent in the liquid and P degree is the vapor pressure of the pure solvent at the same temperature. When a non-volatile solute is dissolved, the solvent mole fraction falls below one and the solution vapor pressure drops by the colligative amount Delta P equals P degree minus P, which is proportional to the solute mole fraction. Students use the relation for homework on ideal binary mixtures and vapor-pressure lowering drills; lab instructors check freeze-and-boil colligative experiments against the same closed form; process engineers estimate how much a dilute non-volatile load reduces the vapor pressure of water, ethanol, or organic solvents before running a full thermo package. This calculator accepts solvent mole fraction and pure-solvent vapor pressure, returns the solution vapor pressure with solute mole fraction, vapor-pressure lowering, unit labels, and the formula string for audit, and rejects a mole fraction outside zero to one so a non-physical composition never becomes a silent scaled pressure.
How Raoult's law defines solution vapor pressure
Start from the ideal-solution statement taught in general and physical chemistry: the partial vapor pressure of a volatile component equals its liquid mole fraction times the pure-component vapor pressure at the same temperature. For a single solvent with a non-volatile solute the only vapor contributor is the solvent, so the measurable solution vapor pressure collapses to P equals x solvent times P degree solvent.
P degree solvent is the vapor pressure of pure solvent at the temperature of interest; its numerical unit is whatever you choose for the problem—torr, millimeters of mercury, kilopascals, bar, or atmospheres—and the product P is returned in the same unit without conversion. When the solute is non-volatile the vapor-pressure lowering is Delta P equals P degree minus P, which equals x solute times P degree with x solute equals one minus x solvent.
Where Raoult's law vapor pressure calculators show up in real work
Secondary-school and introductory university chemistry courses assign vapor-pressure lowering drills that multiply a tabulated pure-solvent vapor pressure by a stated solvent mole fraction; a single trusted oracle speeds grading and demo setup without hand-waving floating-point noise. Laboratory courses measure vapor pressure or boiling behavior of aqueous and organic mixtures and compare experimental lowering to the ideal Raoult prediction, so students need the same closed form their report cites.
Process and formulation engineers estimate how much a non-volatile additive reduces solvent vapor pressure when designing drying, distillation, or emission-control steps before invoking a full activity-coefficient package. Environmental and occupational-hygiene workflows convert liquid composition into an expected partial pressure for volatile organics under the ideal-solution assumption as a first-order screen.
If you later need multi-component Raoult sums for several volatile solvents, Antoine-based pure vapor pressures from temperature, boiling-point elevation, or mole fraction from component moles, treat those as separate search intents and landings rather than overloading this Raoult's law vapor pressure calculator page.
What you can do with it
Homework and exam answer keys
Compute P = x × P° for textbook ideal-solution sets so students and auto-graders share one closed-form Raoult ground truth.
Lab vapor-pressure lowering checks
Predict ideal solution vapor pressure and ΔP from measured solvent mole fraction before comparing to experimental manometer data.
Process screening of non-volatile loads
Estimate how much a dilute non-volatile additive reduces solvent vapor pressure for drying, distillation, or emission first-pass sizing.
FAQ
What mole fraction range is accepted?
Solvent mole fraction must be between 0 and 1 inclusive.
What pressure units can I use?
Any consistent pressure unit—torr, mmHg, kPa, bar, or atm.
Does this handle multi-component volatile solvents?
No. This capability applies the single-solvent form P = x × P°.
What does it cost?
The browser tool is free and runs locally.
Why must the solute be non-volatile?
Because then only the solvent escapes into the vapor, so the solution's vapor pressure is simply its mole fraction times the pure-solvent value. If the solute evaporated too, its own partial pressure would add to the total and this formula would no longer be enough.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/chem/raoult-law-vapor-pressure \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"mole_fraction":0.85,"pure_vapor_pressure":23.8}'const res = await fetch("https://api.kit.forhosting.com/chem/raoult-law-vapor-pressure", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"mole_fraction": 0.85,
"pure_vapor_pressure": 23.8
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/raoult-law-vapor-pressure",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"mole_fraction": 0.85,
"pure_vapor_pressure": 23.8
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/raoult-law-vapor-pressure", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"mole_fraction":0.85,"pure_vapor_pressure":23.8}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"mole_fraction":0.85,"pure_vapor_pressure":23.8}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/raoult-law-vapor-pressure", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"mole_fraction": 0.85,
"pure_vapor_pressure": 23.8
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.raoult_law_vapor_pressure",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
How Raoult's law defines solution vapor pressure
Here x solvent is the solvent mole fraction, a dimensionless quantity between zero and one inclusive; zero means no solvent remains in the liquid, one is the pure-solvent limit, and values outside that closed interval are non-physical for a mole fraction and fail validation with a clear invalid_input message. The capability evaluates the closed form in deterministic floating-point arithmetic: no random seeds, no wall-clock timestamps, no network calls, and no model drift, so identical inputs always produce identical JSON. It does not sum multi-component volatile solvents, look up Antoine coefficients for pure vapor pressures, convert pressure units, or compute boiling points; those are separate search intents with their own landings when product coverage grows.
What you send and what you get back
Classic textbook checks include water at about twenty-three point eight torr with solvent mole fraction zero point eight five giving about twenty point two three torr, pure solvent with x equals one reproducing P degree exactly, and a fully soluted limit with x equals zero giving zero vapor pressure. Send two required finite numbers under the fields mole_fraction and pure_vapor_pressure. mole_fraction is the solvent mole fraction x solvent and must lie in the closed interval from zero to one inclusive; values below zero or above one fail because a mole fraction outside that range is non-physical. pure_vapor_pressure is P degree of the pure solvent at your temperature and must be non-negative; zero is the non-volatile pure-solvent limit and yields a solution vapor pressure of zero. Missing fields, non-numeric strings, NaN, Infinity, mole fractions outside zero to one, and negative pure vapor pressure fail with invalid_input and are not charged. On success the response echoes cleaned mole_fraction and pure_vapor_pressure, provides vapor_pressure as the primary Raoult product, includes solute_mole_fraction as one minus x solvent, vapor_pressure_lowering as P degree minus P, a units hint object, method raoults_law, and formula P equals x solvent times P degree solvent so homework write-ups can cite the same expression. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance.
Where Raoult's law vapor pressure calculators show up in real work
Because this capability is a pure parse engine, API pricing is the published base of $0.002 per successful item and failed validation does not consume balance.
Technical questions
What is Raoult's law for vapor pressure?
For an ideal solution the solvent vapor pressure is P = x_solvent × P°_solvent, where x_solvent is the liquid mole fraction of solvent and P° is the pure-solvent vapor pressure at the same temperature.