Vaporization enthalpy calculator
Get a liquid's latent heat of vaporization from two vapor-pressure readings: a pressure that doubles from 1000 to 2000 between 300 K and 310 K gives the enthalpy in J/mol. Pressures can be in any unit, as long as both match. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- T1
- 300
- T2
- 310
- Vapor pressure at t1
- 1,000
- Vapor pressure at t2
- 2,000
- Resistance (Ω)
- 8.314462618 J/mol
- Delta h vap
- 53,597.2607903011
- Precision
- 10
This vaporization enthalpy calculator estimates the molar enthalpy of vaporization ΔHvap from two vapor-pressure readings at two absolute temperatures using the integrated two-point Clausius–Clapeyron equation. When vapor pressure roughly doubles over a modest temperature window, the slope of ln P versus one over T encodes the latent heat of vaporization under the constant-ΔHvap idealization common in general chemistry and introductory physical chemistry. The closed form used here is fully deterministic: ΔHvap equals R times the natural logarithm of P2 over P1, divided by one over T1 minus one over T2, with temperatures in kelvin and pressures in any consistent unit. Classic textbook checks include paired saturation points for water or organic solvents where tabulated vapor pressures at two temperatures recover tens of kilojoules per mole. The engine uses only finite arithmetic with no network calls, no sampling, and no date-dependent shortcuts.
What two vapor-pressure readings tell you
The Clausius–Clapeyron relation connects the vapor-pressure curve of a pure substance to its enthalpy of vaporization. Under the common approximation that ΔHvap is constant over a limited temperature interval and that vapor behaves ideally with negligible liquid molar volume, integrating between two saturation points yields ln of P2 over P1 equals minus ΔHvap over R times the difference of reciprocal absolute temperatures. Solving for the latent heat produces ΔHvap equals R times ln of P2 over P1, all divided by one over T1 minus one over T2.
Absolute temperatures must be in kelvin; the two vapor pressures may be in pascals, atmospheres, torr, bar, or any other unit as long as both share that unit, because only their ratio enters the logarithm.
Where vaporization enthalpy from vapor pressure fits in study, lab, and software
Students meet the two-point Clausius–Clapeyron rearrangement when a textbook table lists vapor pressures at two temperatures and asks for ΔHvap, when a lab measures saturation pressures in a closed apparatus and needs a single latent-heat estimate, or when a contest problem gives paired P and T values without a full Antoine fit. Teachers can pre-generate answer keys for modest temperature windows and positive pressures without trusting opaque calculator apps that silently convert Celsius without adding 273.15 or mix pressure units inside the logarithm.
Developers call the same contract from scripts to seed deterministic test vectors for physical-chemistry education products, to normalize user-entered vapor-pressure pairs before plotting ln P versus one over T, or to satisfy programming-challenge statements that demand ΔHvap rather than a hand-waved slope. Process and quality workflows sometimes need a fast offline estimate of latent heat from two plant vapor-pressure readings so dilute checks can be validated without opening a full thermodynamics package.
Product and tutoring tools use the closed form when a learner types two temperatures and two pressures and the UI must show a clean joules-per-mole result with a formula echo.
What you can do with it
Homework and lab ΔHvap checks
Estimate molar enthalpy of vaporization from two textbook or measured vapor-pressure points at two absolute temperatures before writing lab reports.
Tutoring and education software
Normalize learner-entered P–T pairs into a single latent-heat result with a clear formula echo for interactive lessons.
FAQ
What does the vaporization enthalpy calculator cost?
It is free in your browser on this page.
What formula is used for ΔHvap?
The two-point integrated Clausius–Clapeyron form: ΔHvap = R · ln(P2/P1) / (1/T1 − 1/T2), with T1 and T2 in kelvin and P1, P2 in the same pressure unit.
What happens if the two temperatures are equal?
Equal temperatures make the reciprocal-temperature denominator zero, so enthalpy of vaporization cannot be computed from a single temperature.
Which units should I use for pressure and temperature?
Temperatures must be absolute kelvin. Pressures may be any consistent unit (Pa, atm, torr, bar, …) because only the ratio P2/P1 appears.
Why are results rounded to a precision?
Default precision is 10 decimal places; you can set precision from 0 to 15.
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/clausius-clapeyron-enthalpy \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"t1":300,"t2":310,"p1":1000,"p2":2000}'const res = await fetch("https://api.kit.forhosting.com/chem/clausius-clapeyron-enthalpy", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"t1": 300,
"t2": 310,
"p1": 1000,
"p2": 2000
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/clausius-clapeyron-enthalpy",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"t1": 300,
"t2": 310,
"p1": 1000,
"p2": 2000
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/clausius-clapeyron-enthalpy", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"t1":300,"t2":310,"p1":1000,"p2":2000}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"t1":300,"t2":310,"p1":1000,"p2":2000}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/clausius-clapeyron-enthalpy", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"t1": 300,
"t2": 310,
"p1": 1000,
"p2": 2000
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.clausius_clapeyron_enthalpy",
"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.
Limits
max_abs_value | 1000000000000000 |
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
What the two-point Clausius–Clapeyron result means and what the response contains
This calculator returns the normalized t1, t2, p1, p2, and r values, the enthalpy delta_h_vap rounded to the requested precision, the precision used, the method tag clausius_clapeyron_enthalpy, a short formula reminder, and the unit string J/mol when the default gas constant in joules per mole-kelvin is used. There is no approximate CAS path and no random sampling of the vapor-pressure curve: the engine either evaluates the closed form inside the supported magnitude range or rejects the input with invalid_input. Rejected cases include missing temperatures or pressures, non-finite values, magnitudes above the documented ceiling, non-positive temperatures or pressures, a zero gas constant, and any pair of equal temperatures, because equal T makes the reciprocal-temperature denominator zero and enthalpy of vaporization cannot be inferred from a single temperature.
How temperatures, pressures, and the gas constant are read
Optional leading plus signs and scientific notation on numeric strings are allowed when they parse as finite decimals; embedded spaces, hex prefixes, and non-numeric tokens are rejected with a clear message so a bad cell does not become a wrong latent heat. Both vapor pressures must be strictly positive so that the pressure ratio and its logarithm are defined. The preferred fields are t1 and t2 for the two absolute temperatures in kelvin, p1 and p2 for the vapor pressures at those temperatures, and optional r for the gas constant, but aliases T1, temperature1, and temp1 for the first temperature, T2, temperature2, and temp2 for the second, P1, pressure1, and vapor_pressure1 for the first pressure, P2, pressure2, and vapor_pressure2 for the second, and R or gas_constant for r are accepted so spreadsheets, query strings, and typed forms can reuse the same contract. Both temperatures must be strictly positive absolute kelvin values and must differ: if t1 equals t2 within a tiny epsilon, the request fails with invalid_input because the Clausius–Clapeyron denominator vanishes. The default gas constant is 8.314462618 joules per mole-kelvin, which yields delta_h_vap in joules per mole; if your problem states R in kilojoules per mole-kelvin, supply that R and interpret the result in kilojoules per mole consistently. An optional precision field, with aliases decimals and dp, chooses how many decimal places to expose for the numeric fields, defaulting to ten and capped at fifteen so JSON goldens stay stable. Pair the free browser run on this page with the priced API when you need the identical mathematical result inside an automated pipeline without re-implementing the two-point latent-heat formula yourself.
Where vaporization enthalpy from vapor pressure fits in study, lab, and software
On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split — free interactive exploration and a priced, identical API — matches the rest of the KIT tier-A catalog and lets you move from a classroom demo to an automated pipeline without changing the mathematical result. Pair it with gas-law or colligative tools when you need related chemistry steps, and with other chem endpoints when you are building a small lab of free, deterministic vapor-liquid helpers.
Integration use cases
Deterministic test fixtures
Generate reproducible delta_h_vap values for unit tests, property checks, and CI assertions on Clausius–Clapeyron formulas.