Trouton's rule calculator
Water boiling at 373.15 K gives a Trouton estimate of 32837.2 J/mol for its enthalpy of vaporization. Enter a normal boiling point in kelvin and the rule multiplies it by 88 J/(mol·K), a fast ballpark before you reach for tables. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Boiling point
- 373.15
- Boiling point unit
- K J/mol
- Trouton constant
- 88
- Trouton constant unit
- J/(mol·K)
- Entropy of vaporization
- 88
Trouton's rule is a classical empirical relation that estimates the molar enthalpy of vaporization of a pure liquid from its normal boiling point alone. For many non-associating liquids the molar entropy of vaporization at the normal boiling temperature is roughly constant, near eighty-eight joules per mole per kelvin, so the product of that constant and the absolute boiling temperature gives a quick estimate of Delta H sub vap in joules per mole. Students use the rule to check whether a tabulated heat of vaporization is in the right ballpark, and process engineers use it for first-pass heat balances when a precise latent-heat table is not yet available. This calculator takes a single finite positive boiling point in kelvin, multiplies by the textbook Trouton constant of eighty-eight, and returns the estimated molar enthalpy of vaporization with the constant and formula spelled out in the response. Non-positive boiling points are rejected because absolute temperature must be positive for the product to be physically meaningful.
How Trouton's rule estimates enthalpy of vaporization
At the normal boiling point under one atmosphere, many non-polar and weakly polar liquids show a molar entropy of vaporization clustered around the same value. Trouton noticed that this entropy change, Delta S sub vap, is approximately eighty-eight joules per mole per kelvin, which is about ten point five calories per mole per kelvin in older units.
Because the phase change is reversible at equilibrium, the corresponding molar enthalpy of vaporization is the product of that entropy and the absolute boiling temperature: Delta H sub vap approximately equals eighty-eight times T sub b, with T sub b in kelvin and Delta H sub vap in joules per mole.
The estimate is rough: hydrogen-bonded liquids such as water and alcohols often show larger true Delta S sub vap, while some low-boiling or associating species deviate the other way, so treat the result as an order-of-magnitude or first-principles check rather than a substitute for calorimetry or a quality vapor-pressure fit. The arithmetic here is a single multiplication after validating that the boiling point is a finite positive number. Entries of NaN or Infinity are rejected early.
A zero or negative boiling point fails with a message that Trouton's rule requires T sub b greater than zero, so clients can fix units or sign rather than receiving a meaningless negative or zero enthalpy. Optional work such as converting Celsius to kelvin, looking up tabulated latent heats, or inverting Clausius–Clapeyron slopes belongs in separate tools; this page stays focused on the named empirical product students search for when they ask for a Trouton's rule calculator.
Where Trouton's rule calculators show up in real work
Secondary-school and college general chemistry and physical chemistry courses constantly assign Trouton's rule problems that ask students to estimate molar heats of vaporization from listed boiling points and to discuss why water deviates; a single trusted oracle that returns eighty-eight times T sub b speeds grading and demo setup. Process chemists and plant engineers sketch first-pass condenser and reboiler duties for solvents when only a boiling point is known, and they need a transparent product rather than a black-box table lookup alone.
Distillation labs use Trouton estimates to show why latent-heat loads scale with boiling temperature before students open steam tables. Quality and safety reviews sometimes compare a measured latent heat against the Trouton ballpark to flag unit errors or transcription mistakes in datasheets. If you later need molar enthalpy of vaporization from measured heat and moles, Gibbs free energy from enthalpy and entropy, or Clausius–Clapeyron vapor-pressure slopes, treat those as separate search intents and landings rather than overloading this Trouton's rule calculator page.
What you can do with it
Homework and exam answer keys
Estimate ΔH_vap ≈ 88·T_b from textbook boiling points so teachers and auto-graders share one Trouton's rule ground truth.
First-pass heat balances
Sketch condenser or reboiler latent-heat loads for a solvent when only the normal boiling point in kelvin is known.
Spot-check tabulated latent heats
Compare a datasheet ΔH_vap against the Trouton estimate to catch unit mistakes or unrealistic values before scale-up.
FAQ
What is Trouton's rule?
Trouton's rule states that the molar entropy of vaporization of many liquids at the normal boiling point is roughly constant (≈ 88 J/(mol·K)), so ΔH_vap ≈ 88 · T_b with T_b in kelvin and ΔH_vap in J/mol.
What happens if the boiling point is zero or negative?
Absolute temperature must be positive; Trouton's product is undefined or non-physical for T_b ≤ 0.
What units should I use?
Convert Celsius to kelvin before calling if your data are in °C.
Why does water's Trouton estimate differ from the experimental value?
Hydrogen-bonded and associating liquids often have larger true ΔS_vap than 88 J/(mol·K). Trouton's rule is an empirical average for non-associating liquids, not a precision measurement.
What does it cost?
The browser tool is free and runs locally.
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/trouton-rule \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"boiling_point":373.15}'const res = await fetch("https://api.kit.forhosting.com/chem/trouton-rule", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"boiling_point": 373.15
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/trouton-rule",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"boiling_point": 373.15
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/trouton-rule", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"boiling_point":373.15}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"boiling_point":373.15}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/trouton-rule", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"boiling_point": 373.15
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.trouton_rule",
"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 Trouton's rule estimates enthalpy of vaporization
There is no random seed, no wall-clock timestamp, no network call, and no model drift, so identical T sub b always produces identical JSON.
What you send and what you get back
Send one field. The field is required. Keep kelvin throughout a problem set so students never multiply Celsius values by the Trouton constant by mistake. The field boiling_point is a finite positive number for the normal boiling temperature T sub b in kelvin. Missing fields, empty strings, non-numeric strings, NaN, Infinity, and non-positive boiling points fail with invalid_input and are not charged. When boiling_point is zero or negative the request fails with a clear message that Trouton's rule needs a positive absolute boiling point. On success the response echoes the cleaned boiling_point with boiling_point_unit equal to K, the trouton_constant of eighty-eight with trouton_constant_unit equal to J per mol per K, the entropy_of_vaporization equal to that same constant, the enthalpy_of_vaporization equal to eighty-eight times T sub b, the unit string J per mol for the enthalpy, the formula string showing Delta H sub vap approximately eighty-eight times T sub b, and the method trouton_rule. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance.
Where Trouton's rule 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.