Equilibrium partial pressure from Kp calculator
Give Kp, the starting pressure of pure A and the coefficients, and get every partial pressure at equilibrium: for A ⇌ B + C with a Kp of 0.25 and 1 atm of A, the recomputed Kp check comes back at 0.25. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Kp
- 0.25
- Initial pressure
- 1
- Reactant coefficient
- 1
- Product coefficients
- 1, 1
- Kp
- 0.25
- P0
- 1
- A
- 1
- Extent
- 0.3903882032
An equilibrium partial pressure from Kp calculator turns the gas-phase equilibrium constant, the starting pressure of pure reactant A, and reaction stoichiometry into the set of partial pressures that satisfy mass action at constant volume and temperature. Enter Kp (non-negative), the initial partial pressure of A, the reactant coefficient a, and the product stoichiometric coefficients; the tool solves for the pressure extent x in P_A equals P0 minus a times x and P_product equals nu times x, then returns every equilibrium partial pressure, the total pressure, a readable reaction string, a recomputed Kp check, and ordered substitution steps you can verify by hand. Homework ICE tables, exam prep for N2O4-style dissociations, process-design sanity checks, and coding kata expected values all need that same deterministic solve without opening a computer-algebra system.
What Kp and partial pressures mean for a A(g) ⇌ products
Starting with pure A at initial pressure P0 and no products, a convenient constant-volume, constant-temperature bookkeeping variable is the pressure extent x equal to xi times RT over V. As the reaction advances, reactant pressure falls and product pressures rise until the mass-action expression equals the given Kp. When Kp is zero the only consistent solution is x equals zero (all reactant remains).
When Kp is large the system approaches complete conversion of A and product pressures dominate. Negative Kp is never a valid target for this calculator: equilibrium constants built from partial pressures of ideal gases are non-negative for the irreversible-looking complete-reactant limit and positive for any finite equilibrium with products present. Teachers and automated graders care about that rejection as much as the numbers: a silent absolute-value clamp would hide a sign error in a student’s Kp.
Practical tips, limits, and related searches
Keep Kp non-negative and keep P0 and all stoichiometric coefficients strictly positive so the pressure-extent interval is non-empty and the mass-action expression is defined. Prefer sending numbers rather than pre-formatted strings when you control the client; string numbers are accepted only when they match a strict finite numeric pattern.
Remember that this landing targets people searching for an equilibrium partial pressure from Kp calculator, gas-phase ICE table from Kp, or partial pressures at equilibrium online; it is not a Kc-from-concentration Ostwald tool, not a reaction-quotient Q checker for arbitrary current compositions, and not a multi-reactant solver with products already present at t equals zero. Prefer this capability when you have Kp, a pure-A initial pressure, and simple product stoichiometry; if several reactants start with nonzero pressures, build a general multi-species ICE table elsewhere.
What you can do with it
Homework ICE tables for gas equilibria
Students enter Kp, P0, and stoichiometry from textbook problems and compare hand-written partial pressures and total pressure against a deterministic result before submitting.
Quick design sanity checks
Process engineers approximate pure-feed gas equilibria at fixed V,T from a literature Kp and starting pressure without rebuilding a full process simulator case.
FAQ
What happens when Kp is negative?
Gas-phase mass-action Kp values used here must be non-negative; a negative constant is outside the model domain.
Are products allowed to have nonzero initial pressure?
Not in this model. Use a general multi-species ICE solver if products are already present.
Which pressure unit should I use?
Any consistent pressure unit (bar, atm, Pa, …) as long as Kp was defined with the same unit for partial pressures. The response reports partial pressures in that same unit.
Is the equilibrium partial pressure calculator free?
The browser tool is free and runs locally.
What happens when Kp is zero or very large?
With a Kp of zero the reaction does not advance and all the reactant remains. As Kp grows, the system approaches complete conversion of A and the product pressures dominate the mixture.
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/equilibrium-partial-pressure \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"kp":0.25,"initial_pressure":1,"reactant_coefficient":1,"product_coefficients":[1,1]}'const res = await fetch("https://api.kit.forhosting.com/chem/equilibrium-partial-pressure", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"kp": 0.25,
"initial_pressure": 1,
"reactant_coefficient": 1,
"product_coefficients": [
1,
1
]
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/equilibrium-partial-pressure",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"kp": 0.25,
"initial_pressure": 1,
"reactant_coefficient": 1,
"product_coefficients": [
1,
1
]
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/equilibrium-partial-pressure", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"kp":0.25,"initial_pressure":1,"reactant_coefficient":1,"product_coefficients":[1,1]}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"kp":0.25,"initial_pressure":1,"reactant_coefficient":1,"product_coefficients":[1,1]}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/equilibrium-partial-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
{
"kp": 0.25,
"initial_pressure": 1,
"reactant_coefficient": 1,
"product_coefficients": [
1,
1
]
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.equilibrium_partial_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.
Limits
max_abs | 1000000000000 |
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
Negative Kp is rejected with a clear invalid_input error because a negative gas-phase mass-action constant is outside the physical domain of this model.
What Kp and partial pressures mean for a A(g) ⇌ products
For an ideal gas reaction written a A goes to products with stoichiometric coefficients b_j for each product species B_j, the pressure-based equilibrium constant Kp is the mass-action product of partial pressures raised to stoichiometric powers: Kp equals the product over products of P_Bj to the b_j, divided by P_A to the a, when each partial pressure is expressed in the same pressure unit you used to define the standard state (often bar). Then P_A equals P0 minus a times x, each product partial pressure equals b_j times x, and the total pressure is the sum of those partial pressures.
How to call the API and what comes back
Kp must be finite and non-negative; initial pressure and the reactant coefficient must be strictly positive; every product coefficient must be strictly positive. Send a JSON body with four required fields: kp (aliases Kp, K_p, equilibrium_constant, k_p), initial_pressure (aliases P0, p0, P_initial, pressure, P, p_initial), reactant_coefficient (aliases a, nu_reactant, nu_A, reactant_nu, reactant_stoich, reactant_coeff), and product_coefficients (aliases product_nu, product_stoich, nu_products, product_coeff, products_nu)—a non-empty array of strictly positive stoichiometric coefficients. Absolute values must stay within the published limit of one trillion so intermediate powers remain usable in IEEE double precision on the Worker and in the browser. On success the response echoes rounded kp and Kp, initial_pressure and P0, reactant_coefficient and a, product_coefficients, extent and x for the solved pressure extent, partial_pressures with reactant, A, products array, and labeled B, C, … fields, total_pressure, kp_check recomputed from the same partial pressures, reaction and formula strings, unit_pressure noting that the unit matches your input, message, and steps as an ordered array of substitution lines. Failed validation—missing fields, non-finite numbers, negative Kp, non-positive initial pressure or coefficients, empty product list, or values outside absolute limits—returns invalid_input and is never charged. Pricing is a flat $0.002 per successful request with unit item and zero marginal unit price.
Practical tips, limits, and related searches
When teaching, pair examples: Kp equals zero point two five with P0 equals one, a equals one, and product coefficients one and one models A reversible B plus C and yields a moderate conversion; Kp equals four with A reversible two B (product coefficients array of a single two) shows the squared product pressure; and a deliberately negative Kp demonstrates the hard invalid_input path required by this product. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when learners deliberately send a negative Kp.
Integration use cases
Unit tests for student equilibrium solvers
Generate known-good A ⇌ B + C and A ⇌ 2B cases plus deliberate negative-Kp rejections for coding kata expected values.
Technical questions
What does this calculator compute from Kp?
It solves for the constant-V,T pressure extent of a A(g) ⇌ products starting with pure A at initial_pressure, then returns every equilibrium partial pressure, total pressure, and a recomputed Kp check.