Gas collected over water calculator
Correct a wet-gas reading for water vapor: 750 torr total with 23.8 torr of water vapor leaves 726.2 torr of dry gas, ready for the ideal gas law. The mole fractions of vapor and dry gas come too. Free in your browser.
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Worked example
- Total pressure
- 750
- Water vapor pressure
- 23.8
- Dry gas pressure
- 726.2
- Vapor fraction
- 0.0317333333
- Dry gas fraction
- 0.9682666667
When a gas is generated in the laboratory and trapped by displacement of water, the volume above the liquid is saturated with water vapor. The manometer or barometer reading is therefore the sum of two partial pressures: the dry gas you care about and the equilibrium vapor pressure of water at the collection temperature. Dalton's law of partial pressures rearranges to the standard textbook correction P gas equals P total minus P H2O. Students apply the formula before plugging into the ideal-gas law to find moles of hydrogen, oxygen, or nitrogen from eudiometer volumes; lab instructors check report tables against the same closed form; analytical and process chemists convert wet-stack or wet-gas meter readings into dry partial pressures for mass-balance work. This calculator accepts total wet-gas pressure and water vapor pressure in any consistent unit, returns dry-gas partial pressure with vapor and dry-gas mole fractions, unit labels, and the formula string for audit, and rejects a vapor pressure that exceeds total so a non-physical composition never becomes a silent negative dry-gas result.
How gas collected over water defines dry-gas partial pressure
Start from the laboratory setup taught in general chemistry: a gas evolved from a reaction is collected by water displacement, so the headspace sits in contact with liquid water long enough to reach vapor–liquid equilibrium for H2O. The measured total pressure of that wet gas is then the sum of the partial pressure of the dry product gas and the equilibrium vapor pressure of water at the collection temperature.
Dalton's law of partial pressures states P total equals P gas plus P H2O, which rearranges immediately to the correction used on every homework and lab sheet: P gas equals P total minus P H2O. Here P total is the total wet-gas pressure—often the barometric pressure when the water levels inside and outside the eudiometer are equalized, or a manometer reading when they are not—and P H2O is looked up or given for the temperature of the water bath.
Both quantities share the same pressure unit: torr, millimeters of mercury, kilopascals, bar, or atmospheres.
Where gas-collected-over-water calculators show up in real work
Secondary-school and introductory university chemistry courses assign eudiometer and water-displacement problems that require subtracting tabulated water vapor pressure from barometric pressure before computing moles with PV equals nRT; a single trusted oracle speeds grading and demo setup without hand-waving floating-point noise. Laboratory courses that generate hydrogen from metals and acid, oxygen from peroxide, or nitrogen from azides collect product over water and need the same dry-gas partial pressure their reports cite.
Environmental and stack-sampling workflows convert wet-gas meter or pitot readings into dry partial pressures when water vapor is present at a known temperature. Process and analytical chemists apply the correction when metering humid process streams or calibrating gas volumes against dry standards. If you later need water vapor pressure from temperature, multi-component Dalton sums from mole fractions, or moles from pressure–volume–temperature with the dry-gas pressure already corrected, treat those as separate search intents and landings rather than overloading this gas collected over water calculator page.
What you can do with it
Homework and exam answer keys
Compute P_gas = P_total − P_H2O for textbook water-displacement sets so students and auto-graders share one closed-form dry-gas ground truth.
Eudiometer and lab report checks
Correct measured wet-gas pressure for water vapor before ideal-gas mole calculations in metal–acid hydrogen labs and similar collection experiments.
Wet-gas meter and stack sampling
Convert humid total pressure readings into dry-gas partial pressure when vapor pressure at the sample temperature is known.
FAQ
What is the formula for gas collected over water?
By Dalton's law, P_gas = P_total − P_H2O, where P_total is the total wet-gas pressure and P_H2O is the water vapor pressure at the collection temperature.
What happens if water vapor pressure exceeds total pressure?
A wet-gas headspace cannot have P_H2O greater than P_total; the calculator never returns a negative dry-gas pressure.
What pressure units can I use?
Any consistent pressure unit—torr, mmHg, kPa, bar, or atm.
Does this look up water vapor pressure from temperature?
No. You supply P_H2O yourself (from a table, Antoine equation, or problem statement). Temperature-based vapor-pressure lookup is a separate search intent.
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/gas-collected-over-water \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"total_pressure":750,"water_vapor_pressure":23.8}'const res = await fetch("https://api.kit.forhosting.com/chem/gas-collected-over-water", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"total_pressure": 750,
"water_vapor_pressure": 23.8
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/gas-collected-over-water",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"total_pressure": 750,
"water_vapor_pressure": 23.8
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/gas-collected-over-water", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"total_pressure":750,"water_vapor_pressure":23.8}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"total_pressure":750,"water_vapor_pressure":23.8}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/gas-collected-over-water", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"total_pressure": 750,
"water_vapor_pressure": 23.8
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.gas_collected_over_water",
"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 gas collected over water defines dry-gas partial pressure
The calculator performs a single deterministic subtraction with no unit conversion, no random seeds, no wall-clock timestamps, and no network calls, so identical inputs always produce identical JSON. It does not look up vapor pressure from temperature via Antoine coefficients or steam tables, convert torr to kPa, or finish the ideal-gas mole calculation; those remain separate search intents with their own landings when product coverage grows.
What you send and what you get back
Classic textbook checks include seven hundred fifty torr total with twenty-three point eight torr water vapor giving seven hundred twenty-six point two torr dry gas, equal total and vapor pressures giving zero dry gas, and pure dry gas with zero vapor pressure reproducing total pressure exactly. Send two required finite numbers under the fields total_pressure and water_vapor_pressure. total_pressure is the total pressure of the wet gas mixture at collection and must be non-negative; zero is allowed and pairs with zero vapor pressure to yield a zero dry-gas result. water_vapor_pressure is the equilibrium vapor pressure of water at the collection temperature, also non-negative, and must not exceed total_pressure—when vapor pressure is larger than total, the call fails with invalid_input because that composition is non-physical for a wet-gas headspace. Missing fields, non-numeric strings, NaN, Infinity, negative pressures, and vapor above total all fail with invalid_input and are not charged. On success the response echoes cleaned total_pressure and water_vapor_pressure, provides dry_gas_pressure as the primary Dalton result, includes vapor_fraction as P H2O over P total and dry_gas_fraction as P gas over P total (both zero when total is zero), a units hint object, method gas_collected_over_water, and formula P gas equals P total minus P H2O 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 gas-collected-over-water 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.