Enthalpy of vaporization calculator
Get a liquid's molar enthalpy of vaporization from the heat it absorbed and the moles that turned to vapor: 40.7 kJ for 1 mol of water near 100 °C gives 40.7 kJ/mol, labelled endothermic. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Heat absorbed
- 40.7
- Moles
- 1
- Enthalpy of vaporization
- 40.7
- Process
- endothermic
The molar enthalpy of vaporization, often written Delta H sub vap or Delta H sub vaporiz, is the enthalpy change when one mole of liquid becomes vapor under stated conditions, typically near the boiling point at constant pressure. In the laboratory you rarely vaporize exactly one mole; instead you measure the heat q absorbed by the liquid–vapor process for a known amount of substance n and normalize: Delta H sub vap equals q divided by n. A positive result means the vaporization is endothermic—the usual case—and the surroundings cool or heat must be supplied; a negative result would mean net heat release, which is uncommon for pure liquid-to-vapor phase change but can appear if you enter condensation heat as a signed value. This calculator takes those two finite numbers—heat and moles—returns the molar enthalpy of vaporization with a clear thermal process label, and rejects zero moles because dividing by zero would invent a meaningless molar quantity. Keep energy units consistent: if q is in kilojoules then Delta H sub vap is in kilojoules per mole; if q is in joules the answer is joules per mole.
How molar enthalpy of vaporization is defined and computed
Start from a vaporization experiment in which a measured amount of pure liquid is converted to vapor under constant-pressure conditions that approximate the phase-change process of interest. Calorimetry, steam-table energy balances, or an equivalent heat measurement give the heat q associated with that event. Chemistry textbooks and data tables report the intensive quantity per mole of substance vaporized, so you divide: Delta H sub vap equals q over n, where n is the moles of liquid that actually vaporized.
The sign convention used here is the system convention of chemical thermodynamics: heat absorbed by the vaporizing system is positive, so water near one hundred degrees Celsius shows a large positive Delta H sub vap of about forty point seven kilojoules per mole when roughly that heat is supplied per mole of steam formed. The arithmetic is a single division after validating that both fields are finite numbers and that n is strictly greater than zero. Entries of NaN or Infinity are rejected early.
A zero or non-positive mole amount fails with a message that molar enthalpy of vaporization is undefined when n equals zero, so clients can fix the stoichiometry rather than receiving Infinity or a silent NaN. Optional downstream work such as converting grams to moles, reconstructing q from mass times latent heat tables, or fitting Clausius–Clapeyron slopes from vapor-pressure data belongs in separate tools; this page stays focused on the normalizing step that students and lab notebooks actually ask for by name when they search for an enthalpy of vaporization calculator.
What you send and what you get back
Send two fields. The field heat is a finite number for the heat q absorbed during vaporization, in any consistent energy unit such as joules or kilojoules. The field moles is a finite positive number for the amount of substance vaporized in moles.
Both fields are required. Classic textbook checks include vaporizing one mole of water with a measured heat of forty point seven kilojoules to obtain forty point seven kilojoules per mole (endothermic), and absorbing eighty one point four kilojoules while vaporizing two moles to obtain the same molar value. Keep the same energy unit family throughout a problem set so students never mix joules per mole with kilojoules per mole on the same line of working.
Where enthalpy of vaporization calculators show up in real work
Secondary-school and college general chemistry courses constantly assign enthalpy of vaporization problems that start from a measured heat load and a weighed liquid sample converted to moles; a single trusted oracle that returns Delta H sub vap and the endothermic label speeds grading and demo setup. Process chemists and plant engineers record steam and solvent evaporation loads during scale-up, and they need a transparent molar value for heat-balance sheets rather than a black-box table lookup alone.
Distillation and drying labs use measured heater energy and mass of distillate to estimate effective molar heats of vaporization for student notebooks. Quality labs that already know q from instrument software still need a deterministic divide-and-label step for LIMS exports and CAPA reports when batch vaporization amounts change.
If you later need only amount-scaled reaction heat Q equals n times Delta H, molar heat of solution from dissolution calorimetry, or Gibbs free energy from enthalpy and entropy, treat those as separate search intents and landings rather than overloading this enthalpy of vaporization calculator page.
What you can do with it
Homework and exam answer keys
Convert measured vaporization heat and moles of liquid vaporized into ΔH_vap with an endothermic or exothermic label for consistent grading.
Lab notebook normalization
Turn calorimeter or heater heat and weighed moles of distillate into a molar enthalpy of vaporization for reports and heat-balance sheets.
Distillation and drying demos
Show students why boiling requires continuous heat input from the sign and magnitude of ΔH_vap = q/n for water or organic solvents.
FAQ
What is the molar enthalpy of vaporization?
It is the enthalpy change when one mole of liquid becomes vapor under the stated conditions. From an experiment you compute ΔH_vap = q / n, where q is the heat absorbed and n is the moles vaporized.
What does it cost?
The browser tool is free and runs locally.
Why is zero moles not accepted?
Because a heat per mole cannot exist without any moles. Instead of returning infinity, the calculator stops with a clear message so you can fix the amount of substance before the result goes into a report.
Can the molar enthalpy of vaporization come out negative?
Only if you enter a heat release. Vaporizing a pure liquid normally absorbs heat, so the result is positive; a negative value usually means a condensation heat was typed in with its sign.
Which unit does the result come out in?
The unit of your heat per mole of substance vaporized. Kilojoules give kilojoules per mole and joules give joules per mole, so the answer follows whatever unit your measurement used.
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/enthalpy-of-vaporization-moles \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"heat":40.7,"moles":1}'const res = await fetch("https://api.kit.forhosting.com/chem/enthalpy-of-vaporization-moles", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"heat": 40.7,
"moles": 1
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/enthalpy-of-vaporization-moles",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"heat": 40.7,
"moles": 1
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/enthalpy-of-vaporization-moles", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"heat":40.7,"moles":1}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"heat":40.7,"moles":1}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/enthalpy-of-vaporization-moles", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"heat": 40.7,
"moles": 1
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.enthalpy_of_vaporization_moles",
"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 molar enthalpy of vaporization is defined and computed
If the measured heat is essentially zero within floating-point noise, the calculator labels the process thermal_neutral rather than inventing a direction. There is no random seed, no wall-clock timestamp, no network call, and no model drift, so identical heat and moles always produce identical JSON.
What you send and what you get back
Missing fields, empty strings, non-numeric strings, NaN, Infinity, and non-positive moles fail with invalid_input and are not charged. When moles is zero the request fails with a clear message that Delta H sub vap equals q over n is undefined for n equals zero. On success the response echoes the cleaned heat and moles, the molar enthalpy of vaporization enthalpy_of_vaporization, a process label of endothermic, exothermic, or thermal_neutral, the unit string energy per mol reminding you to keep energy units consistent with your choice of q, the formula string Delta H sub vap equals q over n, and the method enthalpy_of_vaporization_moles. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance.
Where enthalpy of vaporization 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 happens if moles is zero?
The request fails with invalid_input. Molar enthalpy of vaporization is undefined when n = 0 because the formula divides by the amount vaporized.
How do I know if vaporization is endothermic or exothermic?
Positive enthalpy_of_vaporization means endothermic (system absorbs heat)—the usual case for liquid → vapor. Negative means exothermic. Near-zero results are labeled thermal_neutral.
What units should I use?
Keep energy units consistent. If heat is in kJ, enthalpy_of_vaporization is in kJ/mol; if heat is in J, the answer is in J/mol. moles must be in mol.