Limiting reactant calculator
Find the reactant that stops the reaction and the leftover of the other: 5 mol of hydrogen with coefficient 2 against 3 mol of oxygen with coefficient 1 makes hydrogen limiting and leaves 0.5 mol of oxygen in excess. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Amount a
- 5
- Coeff a
- 2
- Amount b
- 3
- Coeff b
- 1
- Ratio a
- 2.5
- Ratio b
- 3
- Extent
- 2.5
- Limiting
- A
In any balanced chemical reaction, reactants are consumed in fixed stoichiometric ratios. When the starting amounts are not perfectly matched, one reactant is fully consumed first and stops the reaction; that species is the limiting reactant, and the other is left in excess. Use it for homework checks, lab prep, process mass balance, and any pipeline that needs a deterministic limiting-reagent answer without spreadsheets or ad-hoc scripts.
How the limiting-reactant test works
For a reaction written with stoichiometric coefficients ν_A and ν_B, each available amount is converted into a reaction-extent ratio by dividing amount by coefficient: ratio_A equals n_A over ν_A and ratio_B equals n_B over ν_B. The smaller ratio is the maximum number of reaction units you can complete before something runs out; that side is limiting. The other reactant is excess, and the leftover amount equals its starting amount minus its coefficient times the shared extent.
When the two ratios are equal, both reactants are depleted together, so the tool reports limiting as both and excess remaining as zero. Amounts must be non-negative finite numbers in consistent units—moles are ideal, but any shared mass unit works if both sides use the same basis. Coefficients must be strictly positive; a zero coefficient is rejected because division by zero is undefined and a zero term is not a real reactant in the balanced equation.
Where teams plug this into real workflows
Chemistry teachers and tutors use the calculator to generate expected limiting reagents and excess leftovers before students work multi-step stoichiometry problems. Laboratory technicians check whether a planned charge of two reagents will leave one component as the bottleneck, then adjust feed ratios before heating or mixing. Process engineers and batch-record systems embed the same check in recipes so a production run cannot claim full conversion when the stoichiometry says one feed was short.
Documentation writers embed live examples so readers can change coefficients without leaving the page.
What you can do with it
Homework and lab prep checks
Compute which of two reactants is limiting and how much excess remains before students or lab partners measure yields against theory.
Batch recipe validation
Verify planned feed amounts against stoichiometric coefficients so production charges do not claim full conversion on a short reactant.
FAQ
How does the limiting reactant calculator decide which reactant runs out first?
It divides each reactant amount by its stoichiometric coefficient and takes the smaller ratio as the reaction extent. The reactant with the smaller ratio is limiting; the other is excess.
What units should I use for the amounts?
Coefficients are dimensionless integers or reals from the balanced equation.
What happens if a stoichiometric coefficient is zero?
A coefficient of zero is undefined for the amount-over-coefficient test and is rejected before any result is returned; failed tasks are not charged.
What if both reactants run out at the same time?
Then the two ratios of amount to coefficient are equal, both reactants are used up together and the result names both as limiting, with nothing left in excess.
Does it give the theoretical yield of product?
No. It names the limiting reactant, the number of reaction units completed and the excess left over. To turn the limiting reactant into an amount of product, use the theoretical yield calculator instead.
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/limiting-reactant \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"amount_a":5,"coeff_a":2,"amount_b":3,"coeff_b":1}'const res = await fetch("https://api.kit.forhosting.com/chem/limiting-reactant", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"amount_a": 5,
"coeff_a": 2,
"amount_b": 3,
"coeff_b": 1
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/limiting-reactant",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"amount_a": 5,
"coeff_a": 2,
"amount_b": 3,
"coeff_b": 1
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/limiting-reactant", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"amount_a":5,"coeff_a":2,"amount_b":3,"coeff_b":1}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"amount_a":5,"coeff_a":2,"amount_b":3,"coeff_b":1}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/limiting-reactant", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"amount_a": 5,
"coeff_a": 2,
"amount_b": 3,
"coeff_b": 1
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.limiting_reactant",
"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
This capability compares the amount-to-coefficient ratios of two reactants, identifies the limiting side, reports the reaction extent in stoichiometric units, and returns the unused remainder of the excess reactant as clean JSON.
How the limiting-reactant test works
The calculation is pure arithmetic with fixed twelve-decimal rounding so browser and API outputs match byte-for-byte on the same input.
Inputs, outputs, and units to send
All four fields are required. A classic check is the water-forming reaction with two moles of hydrogen and one mole of oxygen: five units of A with coefficient two against three units of B with coefficient one yields ratio_A of two point five and ratio_B of three, so A is limiting, extent is two point five, and B remains with one half unit of excess. Convert percentages or grams to a shared mole basis before calling if you need chemically absolute leftovers rather than relative bookkeeping. Send four numbers: amount_a and amount_b for the available quantities of reactants A and B, plus coeff_a and coeff_b for their stoichiometric coefficients from the balanced equation. Amounts must be finite and greater than or equal to zero; coefficients must be finite and strictly greater than zero—zero coefficients raise invalid_input and are not charged. The response echoes the four inputs, both ratios, the extent, the limiting label A B or both, the excess reactant label or null, the numeric excess remaining, and a short formula string for audit logs.
Where teams plug this into real workflows
Because each successful call is a single analytic item priced at $0.002, batch scripts can sweep grids of feed ratios or curriculum problem banks without model cost or subscription overhead. Prefer this endpoint when you need a transparent two-reactant limiting test—not a full multi-species atom balance, yield-with-side-reactions solver, or equilibrium calculator, which need richer chemistry and different inputs.
Integration use cases
Automated grading and demos
Score student stoichiometry answers or power interactive study tools with a deterministic JSON limiting-reactant result.
Technical questions
How much does each API call cost?
$0.002 per successful request. The browser tool above runs free locally; the API draws from your prepaid KIT balance only when the task succeeds.
What if both reactants are present in exact stoichiometric proportion?
Both ratios match, so limiting is reported as both, excess_reactant is null, and excess_remaining is zero—neither side is left over after complete conversion.