Equivalent weight calculator
Get the equivalent weight from molar mass and n-factor in one step: sulfuric acid at 98.079 g/mol with both protons reacting gives 49.0395 g per equivalent. Useful for titration prep and normality worksheets. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Molar mass
- 98.079
- Valence
- 2
- N factor
- 2
- Equivalents per mole
- 2
- Equivalent weight
- 49.0395
- Unit molar mass
- g/mol
An equivalent weight calculator turns molar mass and valence—or the number of equivalents per mole—into the classical equivalent weight used in acid–base and redox stoichiometry. Titration prep, normality conversions, exam checks, and automated graders all need that same EW figure without a spreadsheet or a full chemical database.
What equivalent weight means in acid–base and redox chemistry
Equivalent weight is the mass of a substance that supplies or reacts with one mole of reactive units under a defined reaction—protons for acids and bases, electrons for redox couples, or charge for salts in older normality tables. The defining relation is EW equals M divided by n, where M is the molar mass in grams per mole and n is the number of equivalents contributed by one mole of the substance in that reaction.
For a diprotic acid such as sulfuric acid, n is commonly two when both protons titrate; for hydrochloric acid n is one; for a metal ion that gains or loses two electrons in a half-reaction, n is two. That single ratio lets you convert between mass, normality, and mole-based stoichiometry without re-deriving the balanced equation each time.
Teachers and automated graders care about the zero-valence rejection as much as the happy path: dividing by a blank or zero n is not a physical conversion, and a silent fallback would hide a data-entry error. Classroom examples stay concrete—about 98.079 grams per mole of H2SO4 with n equals two yields roughly 49.04 grams per equivalent—while industrial batch sheets simply change M and n.
The check path recomputes EW with the same fixed rounding used for published fields, so a successful response always ships with steps you can assert in tests or worksheets.
Practical tips, limits, and related searches
Keep molar masses and n-factors inside the published absolute range so EW equals M over n cannot overflow into a non-finite result the solver must reject. 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.
Choose n from the reaction you actually run: the same sulfuric acid has n equals two in a full neutralization and n equals one if only the first proton is used—wrong n is a chemistry mistake, not a calculator bug. Prefer this capability when you already know M and n; if your source is only a formula string, resolve molar mass first and decide n from the reaction.
What you can do with it
Titration prep and normality worksheets
Lab technicians enter tabulated molar mass and the reaction n-factor to get equivalent weight before converting target normality into weigh-out mass.
Homework checks for EW = M / n
Students compute equivalent weight for acids, bases, and redox agents and compare hand-written answers against a deterministic result before submitting.
FAQ
What is equivalent weight?
Equivalent weight is the mass that reacts with one mole of reactive units (protons, electrons, or charge). It is computed as EW = M / n, where M is molar mass and n is the number of equivalents per mole.
What is the n-factor or valence here?
It is the number of equivalents contributed by one mole of substance in the chosen reaction—for example 2 for H2SO4 in complete neutralization, or electrons transferred per formula unit in a redox half-reaction.
Is the equivalent weight calculator free?
The browser tool is free and runs locally.
Why can sulfuric acid have two different equivalent weights?
Because n depends on the reaction you run. In a full neutralization both protons react and n is 2, giving 49.0395 g/eq; if only the first proton is used, n is 1 and the equivalent weight equals the molar mass.
What happens if I enter a valence of zero?
The calculator refuses it, because dividing by zero is not a physical conversion. Values extremely close to zero are treated the same way, so a blank or mistyped n-factor never turns into a misleading result.
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/equivalent-weight \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"molar_mass":98.079,"valence":2}'const res = await fetch("https://api.kit.forhosting.com/chem/equivalent-weight", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"molar_mass": 98.079,
"valence": 2
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/equivalent-weight",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"molar_mass": 98.079,
"valence": 2
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/equivalent-weight", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"molar_mass":98.079,"valence":2}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"molar_mass":98.079,"valence":2}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/equivalent-weight", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"molar_mass": 98.079,
"valence": 2
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.equivalent_weight",
"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
When valence is zero the division is undefined, so the API returns a clear invalid_input error instead of inventing infinity or a silent NaN.
How to call the API and what comes back
Valence must be strictly non-zero; values whose absolute value is at or below one times ten to the minus fifteen are treated as zero and rejected. Send a JSON body with two required finite numbers: molar_mass (aliases M, molecular_weight, mw, molarMass, formula_weight) in grams per mole, and valence (aliases n, n_factor, equivalents_per_mole, eq_per_mole, basicity, acidity) as the dimensionless n-factor. Absolute values must stay within the published limit of one trillion so the quotient remains finite in IEEE double precision on the Worker and in the browser. On success the response echoes rounded molar_mass and valence, mirrors n_factor and equivalents_per_mole to the same n, sets equivalent_weight to the rounded EW, unit_molar_mass to g/mol, unit_equivalent_weight to g/eq, formula to the string EW equals M over n, message to a short English summary, and steps to an ordered array of substitution lines. Failed validation—missing fields, non-finite numbers, values outside absolute limits, a non-finite quotient, or zero valence—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: H2SO4 near 98.079 grams per mole with n equals two; HCl near 36.46 with n equals one; a divalent base with M equals forty and n equals two; and valence equals zero to demonstrate the hard invalid_input path. This landing targets people searching for an equivalent weight calculator, EW equals M over n, or n-factor from molar mass online; it is not a molecular-weight look-up from a chemical name, not a full normality converter with solution volume, and not a balanced redox equation solver. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when learners deliberately set valence to zero. Round-trip checks are easy: multiply the published equivalent_weight by n and confirm it recovers molar_mass within the same twelve-decimal rounding, and confirm that flipping the sign of n flips the sign of EW while preserving magnitude when M is positive.
Integration use cases
Unit tests for stoichiometry helpers
Generate known-good EW pairs plus deliberate zero-valence rejections for coding kata, LMS auto-graders, and CI golden fixtures.
Technical questions
What happens when valence is zero?
Division by zero is undefined. The API returns invalid_input instead of producing infinity or NaN.
Does this look up molar mass from a formula?
No. Provide molar_mass yourself (or resolve it first). This tool only divides M by n and returns equivalent weight with steps.