Excess reactant remaining calculator
Find how much of the excess reactant is left over: 28 g of nitrogen and 10 g of hydrogen making ammonia use up all the nitrogen and leave 4 g of hydrogen unreacted. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Mass a
- 28
- Mass b
- 10
- Molar mass a
- 28
- Molar mass b
- 2
- Coefficient a
- 1
- Coefficient b
- 3
- Moles a
- 1
- Moles b
- 5
- Extent a
- 1
- Extent b
- 1.6666666667
In a balanced chemical equation the reactants are rarely mixed in the exact stoichiometric ratio, so one reactant is fully consumed first and the other remains as excess. This excess reactant remaining calculator converts each reactant mass into moles with its molar mass, divides by the balanced coefficient to get available reaction extents, picks the smaller extent as the true reaction progress, and multiplies the unused moles of the excess species by its molar mass to report leftover mass in grams. Students use it to check ammonia, combustion, and titration stoichiometry homework; lab planners use it to size feeds so a catalyst or expensive reagent is never the leftover; automated grade keys need a deterministic oracle that rejects zero coefficients instead of dividing by zero.
How leftover excess mass is computed
Begin with a balanced reaction written so that reactant A appears with stoichiometric coefficient ν_A and reactant B with ν_B. You supply the available mass of each reactant in grams and the molar mass of each species in grams per mole. Convert mass to moles with n_A equals mass_A divided by M_A and n_B equals mass_B divided by M_B.
The theoretical reaction progress each reactant could support is the extent ξ_A equals n_A divided by ν_A and ξ_B equals n_B divided by ν_B. The actual reaction extent is the minimum of those two values: the reactant that runs out first is the limiting reactant, and the other is the excess reactant. Moles of excess consumed equal ν_excess times the reaction extent.
Leftover moles equal the initial moles of the excess species minus moles consumed. Leftover mass equals leftover moles times the molar mass of the excess species. When the two extents are numerically equal the mixture is stoichiometric, both reactants are fully consumed, and leftover mass is zero.
Masses must be non-negative and molar masses strictly positive.
Where excess-reactant remaining calculators show up in real work
Secondary-school and college general chemistry courses constantly assign limiting-reactant drills that end with how many grams of excess remain; a single trusted oracle that returns leftover mass beside extents and labels speeds grading and demo setup. Process chemists sizing batch feeds want the expensive reagent fully consumed and need leftover mass of the cheaper co-reactant so scrubbers and workups are not oversized. Environmental and combustion problems ask how much oxygen remains after complete fuel burn, which is the same extent-min construction with air as reactant B.
If you later need only product moles from a single reactant via mole ratio, grams-to-moles conversion alone, or full multi-product yield tables, treat those as separate search intents and landings rather than overloading this excess reactant remaining calculator page.
What you can do with it
Homework and exam answer keys
Generate leftover excess mass, limiting labels, and extents for textbook two-reactant sets so teachers and auto-graders share one ground truth.
Lab feed and batch sizing
Confirm which reagent will remain and how many grams are left so workup, scrubbing, and waste estimates match the planned charge.
FAQ
What happens if a coefficient is zero?
The limiting-reactant test divides by each coefficient, so a zero coefficient is invalid and never produces Infinity or NaN.
What if the reactants are in exact stoichiometric ratio?
Both extents match, limiting and excess are reported as neither, and leftover mass is zero because both reactants are fully consumed.
Which units should I use?
Masses in grams (g), molar masses in g/mol, and dimensionless balanced coefficients. Leftover mass is returned in grams.
What does it cost?
The browser tool is free and runs locally.
How does the calculator decide which reactant is limiting?
It converts each mass to moles and divides by that reactant's coefficient. The smaller result is the limiting reactant: in the ammonia example nitrogen gives 1 and hydrogen gives 5 divided by 3, so nitrogen runs out first.
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/excess-reactant-remaining \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"mass_a":28,"mass_b":10,"molar_mass_a":28,"molar_mass_b":2,"coefficient_a":1,"coefficient_b":3}'const res = await fetch("https://api.kit.forhosting.com/chem/excess-reactant-remaining", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"mass_a": 28,
"mass_b": 10,
"molar_mass_a": 28,
"molar_mass_b": 2,
"coefficient_a": 1,
"coefficient_b": 3
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/excess-reactant-remaining",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"mass_a": 28,
"mass_b": 10,
"molar_mass_a": 28,
"molar_mass_b": 2,
"coefficient_a": 1,
"coefficient_b": 3
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/excess-reactant-remaining", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"mass_a":28,"mass_b":10,"molar_mass_a":28,"molar_mass_b":2,"coefficient_a":1,"coefficient_b":3}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"mass_a":28,"mass_b":10,"molar_mass_a":28,"molar_mass_b":2,"coefficient_a":1,"coefficient_b":3}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/excess-reactant-remaining", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"mass_a": 28,
"mass_b": 10,
"molar_mass_a": 28,
"molar_mass_b": 2,
"coefficient_a": 1,
"coefficient_b": 3
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.excess_reactant_remaining",
"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 leftover excess mass is computed
Coefficients must be non-zero because the extent ratio divides by each coefficient; a zero coefficient is rejected with invalid_input rather than producing Infinity or NaN. The entire pipeline is deterministic floating-point arithmetic with no random seeds, wall-clock timestamps, or network calls, so identical inputs always yield identical JSON.
What you send and what you get back
Classic textbook checks include nitrogen and hydrogen for ammonia synthesis with coefficients one and three, and any combustion or neutralization pair with integer molar masses. Send six numeric fields. mass_a and mass_b are the available masses of reactants A and B in grams and must be finite and non-negative. molar_mass_a and molar_mass_b are the molar masses in grams per mole and must be finite and strictly positive. coefficient_a and coefficient_b are the balanced stoichiometric coefficients of A and B and must be finite and non-zero. Missing fields, non-numeric strings, NaN, Infinity, negative masses, non-positive molar masses, and zero coefficients fail with invalid_input and are not charged. On success the response echoes the cleaned inputs, the computed moles_a and moles_b, the per-reactant extents extent_a and extent_b, the reaction_extent actually achieved, labels limiting_reactant and excess_reactant as A, B, or neither when stoichiometric, moles consumed of each species, leftover_moles of the excess, and leftover_mass in grams. Units and a short formula string document the method limiting_excess_stoichiometry for clients and widgets. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance.
Where excess-reactant remaining 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.
Integration use cases
Contest and interview fixtures
Bulk-produce leftover-mass answers for ammonia, combustion, and neutralization pairs and catch zero coefficients with invalid_input.
Technical questions
How is leftover excess mass calculated?
Convert each mass to moles, divide by its stoichiometric coefficient to get extents, take the minimum as reaction progress, then leftover_mass = (n_excess − ν_excess × extent) × M_excess.