Reactant mass needed calculator
To make 13.75 g of a product with a molar mass of 44 g/mol from a reactant of 32 g/mol in a 1:1 ratio, weigh out 10 g of reactant. Enter the target mass, both molar masses and both coefficients. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Product mass
- 13.75
- Reactant molar mass
- 32
- Product molar mass
- 44
- Reactant coeff
- 1
- Product coeff
- 1
- Mole ratio
- 1 g
- Moles product
- 0.3125
- Moles reactant
- 0.3125
- Reactant mass
- 10
Lab prep and stoichiometry homework often start from a desired product mass and ask how many grams of a key reactant you must weigh out when the chemical equation is balanced. The answer is not a single proportion of molecular weights alone: you convert the target product mass into moles with the product molar mass, scale by the reactant-to-product coefficient ratio from the balanced equation, then convert reactant moles back into mass with the reactant molar mass. This capability performs that chain deterministically from five numbers—product mass, both molar masses, and both stoichiometric coefficients—and returns the reactant mass plus the intermediate mole amounts so students, tutors, and pipelines can audit every step. Zero molar masses and zero coefficients are rejected as invalid input because they make the mole conversion undefined.
Inputs, units, and the product–mole–reactant path
Provide the target product mass in grams together with the reactant molar mass and product molar mass in grams per mole. Also send the balanced-equation coefficients of the reactant and the product as positive numbers—typically small integers such as one, two, or three, though non-integer coefficients from scaled equations are accepted when they remain finite and positive.
The engine first computes moles of product as mass divided by product molar mass, multiplies by the mole ratio of reactant coefficient over product coefficient, and multiplies again by the reactant molar mass to obtain reactant mass in grams. That is algebraically identical to scaling the product mass by the reactant-to-product molar-mass ratio and by the same stoichiometric ratio in one expression. Mass must be non-negative; molar masses and coefficients must be strictly greater than zero.
Magnitudes are capped for isolate safety while still covering every ordinary classroom and pilot-plant worksheet.
What you can do with it
Homework reactant-needed drills
Students enter a target product mass, both molar masses, and balanced coefficients, then compare the returned reactant mass and intermediate moles against their paper solution.
Lab prep and scale-up charge estimates
Technicians estimate how many grams of reagent to weigh for a desired product charge under a fixed balanced ratio before running the experiment.
FAQ
What happens if a molar mass or coefficient is zero?
Zero molar mass or zero stoichiometric coefficient would make the mole conversion undefined, so those values are rejected and not charged.
Is this the same as mass of product from reactant?
No. This calculator returns reactant mass in grams from a target product mass. For the reverse direction—product mass from a known reactant sample—use the mass-of-product-from-reactant calculator.
Is the reactant mass needed calculator free?
The browser tool runs free on your device.
Which units should I use?
Coefficients are unitless balanced-equation numbers greater than zero.
Does it tell me which reactant is limiting?
No. It works with one reactant and one product under a fixed balanced ratio. It does not compare several reagents to find the limiting one, and it does not look up molar masses from chemical formulas.
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/reactant-mass-for-product \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"product_mass":13.75,"reactant_molar_mass":32,"product_molar_mass":44,"reactant_coeff":1,"product_coeff":1}'const res = await fetch("https://api.kit.forhosting.com/chem/reactant-mass-for-product", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"product_mass": 13.75,
"reactant_molar_mass": 32,
"product_molar_mass": 44,
"reactant_coeff": 1,
"product_coeff": 1
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/reactant-mass-for-product",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"product_mass": 13.75,
"reactant_molar_mass": 32,
"product_molar_mass": 44,
"reactant_coeff": 1,
"product_coeff": 1
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/reactant-mass-for-product", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"product_mass":13.75,"reactant_molar_mass":32,"product_molar_mass":44,"reactant_coeff":1,"product_coeff":1}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"product_mass":13.75,"reactant_molar_mass":32,"product_molar_mass":44,"reactant_coeff":1,"product_coeff":1}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/reactant-mass-for-product", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"product_mass": 13.75,
"reactant_molar_mass": 32,
"product_molar_mass": 44,
"reactant_coeff": 1,
"product_coeff": 1
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.reactant_mass_for_product",
"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
Inputs, units, and the product–mole–reactant path
Zero molar mass or zero coefficient fails with invalid_input so callers never see infinity or NaN disguised as a result. Aliases such as mass, M_reactant, nu_product, and related names are accepted when documented in the parameter contract. Identical inputs always produce identical JSON because the path is pure arithmetic with no network, no random seed, and no wall clock.
Reading reactant_mass, mole intermediates, and error cases
On success the response echoes the five numeric inputs with stable rounding, reports mole_ratio as reactant coefficient divided by product coefficient, and lists moles_product and moles_reactant so you can show the intermediate table students write on paper. The primary answer is reactant_mass in grams with unit set to g, plus a formula string stating the combined expression and a method label reactant_mass_for_product for downstream branching. Use reactant_mass alone when a UI needs a single headline number; use the mole fields when a worked-solution panel must display each conversion; use mole_ratio when you only need the stoichiometric scale factor derived from the coefficients. Missing any required field, non-finite values, negative mass, zero or negative molar mass, and zero or negative coefficients all fail with invalid_input and are not charged after the engine rejects the request. This calculator does not identify a limiting reactant among several reagents and does not look up molar masses from chemical formulas—pair it with a molar-mass tool if you start from molecular formulas rather than tabulated grams per mole. It also differs from the mass-of-product-from-reactant endpoint: here the contract is product mass in and reactant mass out for the reactant-mass-needed search intent.
API usage, pricing, classroom workflows, and practical limits
Call the free in-browser reactant mass needed calculator for homework checks, lecture demos, and quick lab prep without installing a spreadsheet or a chemistry package. Typical classroom examples include how many grams of oxygen are required to form thirteen point seven five grams of carbon dioxide when coefficients are one-to-one, or twice the reactant coefficient when two moles of reactant are consumed per mole of product. Source inputs follow the same retention rules as other KIT parse items. Send a JSON body with product_mass, reactant_molar_mass, product_molar_mass, reactant_coeff, and product_coeff as finite numbers or numeric strings. Use the prepaid API when a tutoring platform, learning-management plug-in, curriculum generator, or process-engineering notebook needs a stable contract under KIT balance at $0.002 per successful item; validation failures never consume balance after rejection. Authors shipping SDKs can map the response fields directly into a results table with columns for product moles, reactant moles, and reactant mass—the same columns students fill when they apply mass–mole–mass stoichiometry by hand in reverse.
Integration use cases
Tutoring and LMS auto-graders
Lesson builders and quiz engines generate worked reverse-stoichiometry items with a stable JSON contract without embedding a full chemistry CAS.
Technical questions
How do I calculate reactant mass needed for a product mass?
Send product_mass in grams, reactant_molar_mass and product_molar_mass in g/mol, and the balanced reactant_coeff and product_coeff. The response includes reactant_mass and the intermediate mole amounts.