Mass percent to molality calculator
A 10% sodium chloride solution, with a molar mass of 58.44 g/mol, is about 1.901 mol/kg. Convert the weight percent printed on a bottle into the molality that freezing-point and boiling-point formulas ask for. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Mass percent of solute
- 10
- Molar mass
- 58.44
- Molality
- 1.9012852688 mol/kg
- Moles solute per 100g
- 0.1711156742
- Mass solvent g per 100g
- 90
- Mass solvent kg per 100g
- 0.09
This mass-percent-to-molality calculator converts a solution’s solute mass percent by weight and the solute’s molar mass into molality in moles per kilogram of solvent. Bottle labels, reagent certificates, and many industrial specs quote concentration as weight percent, while colligative-property tables, freezing-point depression, boiling-point elevation, and osmotic-pressure formulas almost always need molality. The conversion used here is fully deterministic and density-free: on a one-hundred-gram solution basis, moles of solute equal mass percent divided by molar mass, solvent kilograms equal one hundred minus mass percent over one thousand, and molality is moles over solvent kilograms, which algebraically collapses to one thousand times mass percent over molar mass times one hundred minus mass percent. Classic checks include ten percent sodium chloride with molar mass fifty-eight point four four yielding about one point nine zero one molal, and five percent sucrose with molar mass three hundred forty-two point three yielding a much smaller molality once solvent mass is accounted for. The engine uses only finite arithmetic with no network calls, no sampling, and no date-dependent shortcuts.
What molality means and what you get back
Mass percent by weight is grams of solute per one hundred grams of solution. Molality is moles of solute per kilogram of solvent, not per kilogram of the whole mixture. Those two descriptions are not interchangeable without the solute molar mass: the same weight percent maps to different molalities when the molecular weight changes, and molality stays fixed when temperature expands volume while weight percent of a closed mixture does not.
The standard bridge needs no density: take one hundred grams of solution, treat the mass percent number as grams of solute, leave the remainder as grams of solvent, convert solvent grams to kilograms, divide solute grams by molar mass to obtain moles, and form molality as moles per kilogram of solvent. Algebraically that is molality equals one thousand times mass percent divided by molar mass times one hundred minus mass percent. Zero mass percent with positive molar mass is valid and yields zero molality.
Where mass-percent-to-molality conversion fits in study, lab work, and software
Students meet this conversion when a textbook or bottle label gives weight percent for a salt or sugar solution and a problem then asks for molality before freezing-point depression, boiling-point elevation, or vapor-pressure lowering work written in molal terms. Teachers can pre-generate answer keys for colligative-property homework without trusting opaque apps that silently assume density of one or confuse weight percent with volume percent.
Lab technicians and preparers use the same formula when a protocol requires a stated molality and the stock solution is known only as a mass-percent composition with a published molar mass. Developers call the same contract from scripts to seed deterministic test vectors for chemistry tutors, to normalize concentration fields before a LIMS export, or to satisfy programming-challenge statements that demand moles per kilogram of solvent rather than a hand-waved sketch.
Pair it with percent-to-molarity when you also have density and need moles per litre, with molarity-to-molality when you start from a volumetric concentration, and with other chemistry helpers when you are building a small lab of free, deterministic solution calculators.
What you can do with it
Colligative-property homework
Convert bottle weight percent plus molar mass into mol/kg solvent before freezing-point, boiling-point, or osmotic-pressure exercises.
Lab stock and LIMS normalization
Normalize weight-percent stock labels into molality for protocols and exports that require mol/kg solvent.
FAQ
What does the mass percent to molality calculator cost?
It is free in your browser on this page.
Why is density not required?
Molality is defined on solvent mass, and mass percent already separates solute and solvent masses on a fixed solution basis. Density is only needed when converting weight percent to molarity (volume-based).
What if mass percent is outside 0–100?
Percent must be between 0 and 100; 100% pure solute is also rejected because solvent mass would be zero.
Does zero mass percent work?
Yes. Zero percent with a positive molar mass yields molality zero. Negative percent and pure solute at 100% are rejected.
What is the molality of a 10% sodium chloride solution?
About 1.901 mol/kg. Out of 100 g of solution, 10 g is salt and 90 g is water: divide the 10 g by 58.44 g/mol to get the moles, then divide by 0.09 kg of water.
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/mass-percent-to-molality \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"mass_percent":10,"molar_mass":58.44}'const res = await fetch("https://api.kit.forhosting.com/chem/mass-percent-to-molality", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"mass_percent": 10,
"molar_mass": 58.44
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/mass-percent-to-molality",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"mass_percent": 10,
"molar_mass": 58.44
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/mass-percent-to-molality", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"mass_percent":10,"molar_mass":58.44}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"mass_percent":10,"molar_mass":58.44}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/mass-percent-to-molality", 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_percent": 10,
"molar_mass": 58.44
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.mass_percent_to_molality",
"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_value | 1000000000000000 |
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
What mass percent to molality means and what the response contains
This calculator returns the validated mass percent and molar mass, the computed molality, intermediate moles of solute and solvent mass on the one-hundred-gram basis, the precision used, a method tag mass_percent_to_molality, a short formula reminder, and a unit hint mol per kilogram. There is no automatic unit conversion from parts per million, no temperature tables, and no reverse molality-to-percent path in this capability: the engine either evaluates the closed formula inside the supported numeric range or rejects the input with invalid_input. Rejected cases include missing fields, non-finite values, magnitudes above the documented ceiling, molar mass that is zero or negative, mass percent outside the closed interval from zero to one hundred, and pure solute at one hundred percent where solvent mass would be zero and molality is undefined.
How mass percent and molar mass are read and stay reproducible
Optional leading plus signs and scientific notation on numeric strings are allowed when they parse as finite decimals; embedded spaces and non-numeric tokens are rejected with a clear field-named message. The preferred field for composition is mass_percent, but aliases percent, mass_pct, w_w, percent_w_w, wt_percent, and concentration_percent are accepted so spreadsheets, query strings, and lab forms can reuse the same contract. The value must be a finite number between zero and one hundred inclusive of zero; values below zero or above one hundred fail with invalid_input and an explicit between-zero-and-one-hundred message so a bad cell does not become a silent nonsense molality. At exactly one hundred percent the remainder solvent mass is zero, so the request fails with a clear pure-solute message rather than returning infinity. Molar mass prefers molar_mass with aliases mm, molecular_weight, mw, molarMass, solute_molar_mass, and M, and must be strictly positive in grams per mole because zero molar mass would divide by zero in the moles step. An optional precision field, with aliases decimals and dp, chooses how many decimal places to expose for the numeric outputs, defaulting to ten and capped at fifteen so JSON goldens stay stable. Pair the free browser run on this page with the priced API when you need the identical mathematical result inside an automated pipeline without re-implementing the mass-percent-to-molality formula yourself.
Where mass-percent-to-molality conversion fits in study, lab work, and software
Product and education tools use the endpoint when a learner types mass percent and molar mass and the UI must show molality with a clear unit and intermediate solvent-mass check. On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split — free interactive exploration and a priced, identical API — matches the rest of the KIT tier-A catalog and lets you move from a classroom demo to an automated pipeline without changing the mathematical result.
Integration use cases
Deterministic test fixtures
Generate reproducible molality values from mass percent and molar mass for unit tests, tutors, and CI assertions.
Technical questions
What formula is used?
m = 1000 · w / (MM · (100 − w)), where w is mass percent (w/w) and MM is molar mass in g/mol. The result is molality in mol/kg solvent.