Molality calculator
Half a mole of solute in 1 kg of water is 0.5 mol/kg, and 2 mol in half a kilogram is 4 mol/kg. Get the molality that freezing-point, boiling-point and osmotic work rely on, unaffected by temperature. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Moles solute
- 0.5
- Solvent mass kg
- 1
- Molality
- 0.5
Molality is the concentration of a solution expressed as the amount of solute in moles divided by the mass of pure solvent in kilograms. Unlike molarity, which depends on solution volume and therefore on temperature through thermal expansion, molality is temperature-independent because solvent mass does not change when the liquid expands or contracts. Chemists, pharmacists, and process engineers use molality for colligative-property work such as freezing-point depression, boiling-point elevation, and osmotic calculations where a stable intensive measure is required. Students first meet the definition as m equals n over M of solvent and then drill unit conversions from grams of solvent to kilograms. This calculator accepts moles of solute and solvent mass in kilograms, returns molality in moles per kilogram with unit labels and the formula string for audit, and rejects zero solvent mass so a bad denominator never becomes Infinity or NaN.
How molality is defined and why solvent mass matters
Start from the formal definition used in analytical and physical chemistry textbooks: molality m is the quotient of the amount of solute n measured in moles and the mass of the pure solvent M_solvent measured in kilograms. Write m equals n divided by M_solvent and keep the units mol per kilogram, often spoken as molal. The amount of solute must be finite and non-negative: zero moles is a legitimate empty-solute limit that yields molality zero, while a negative amount is rejected.
Callers who only know grams of solvent must convert to kilograms before calling, because the SI definition of molality uses the kilogram denominator by design. Unlike molarity, which divides moles by litres of solution, molality never needs density or temperature corrections for volume change, which is why colligative-property formulas and many industrial recipes prefer the molal scale.
Where molality calculators show up in real work
Secondary-school and introductory university chemistry courses assign concentration drills that convert grams of solvent to kilograms and divide moles of solute by that mass; a single trusted oracle speeds grading and demo setup without hand-waving floating-point noise. Pharmacy and formulation labs prepare recipes where active ingredient loading is specified on a molal basis so potency stays stable across temperature swings that would change volumetric molarity. Process chemists and chemical engineers size feed ratios for reactors and crystallizers when freeze-point and boil-point tables are tabulated against molality rather than molarity.
Environmental and geochemistry notebooks convert measured dissolved amounts into molal concentrations for aqueous speciation models that expect kg of solvent in the denominator. If you later need molarity from volume, mole fraction, mass percent, freezing-point depression from a cryoscopic constant, or automatic molar-mass conversion from grams of solute, treat those as separate search intents and landings rather than overloading this molality calculator page.
What you can do with it
Homework and lab answer keys
Compute molality m = n / M_solvent from moles of solute and kilograms of solvent so students and auto-graders share one closed-form ground truth.
Colligative-property prep work
Convert batch mole and solvent-mass measurements into molal concentration before applying freezing-point or boiling-point formulas.
Formulation and process recipes
Document temperature-stable molal loadings for pharmaceutical or industrial recipes that must not depend on solution volume.
FAQ
What is molality?
Molality m is the amount of solute in moles divided by the mass of pure solvent in kilograms: m = n / M_solvent, with units mol/kg (molal).
What happens if solvent mass is zero?
Molality divides by solvent mass, so zero (or negative) mass is rejected instead of returning Infinity or NaN.
How is molality different from molarity?
Molarity is moles per litre of solution and changes with temperature as volume expands. Molality is moles per kilogram of solvent and is temperature-independent.
Do I enter solvent mass in grams or kilograms?
Kilograms. Convert grams to kilograms by dividing by 1000 before calling so the result matches the SI molal definition.
What does it cost?
The browser tool is free and runs locally.
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/molality \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"moles_solute":0.5,"solvent_mass_kg":1}'const res = await fetch("https://api.kit.forhosting.com/chem/molality", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"moles_solute": 0.5,
"solvent_mass_kg": 1
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/molality",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"moles_solute": 0.5,
"solvent_mass_kg": 1
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/molality", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"moles_solute":0.5,"solvent_mass_kg":1}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"moles_solute":0.5,"solvent_mass_kg":1}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/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
{
"moles_solute": 0.5,
"solvent_mass_kg": 1
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.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.
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 molality is defined and why solvent mass matters
The solvent mass must be strictly positive; a zero mass would make the ratio undefined and a negative mass is non-physical, so both cases fail validation with a clear invalid_input message rather than a silent Infinity. This capability evaluates the closed form in deterministic floating-point arithmetic: no random seeds, no wall-clock timestamps, no network calls, and no model drift, so identical inputs always produce identical JSON. It does not convert grams of solute to moles, look up molar masses, or compute mole fraction, normality, or molarity; those are separate search intents with their own landings when product coverage grows.
What you send and what you get back
Classic textbook checks include half a mole of solute in one kilogram of water giving molality zero point five, two moles in half a kilogram of solvent giving molality four, and zero moles in any positive solvent mass giving molality zero. Keep mass on the kilogram scale across multi-step lab pipelines: if you weigh solvent in grams, divide by one thousand before calling so the numeric product still matches the molal interpretation your report claims. Send two required finite numbers under the fields moles_solute and solvent_mass_kg. moles_solute is the amount of dissolved species in moles and must be greater than or equal to zero. solvent_mass_kg is the mass of pure solvent in kilograms and must be strictly greater than zero; zero and negative values fail because molality divides by that mass. Missing fields, non-numeric strings, NaN, Infinity, negative moles, and non-positive solvent mass fail with invalid_input and are not charged. On success the response echoes cleaned moles_solute and solvent_mass_kg, provides molality as the primary result in moles per kilogram, includes a units hint object naming moles, kilograms, and moles per kilogram, sets method to molality_from_moles_and_solvent_mass, and formula to m equals n over M_solvent so homework write-ups can cite the same expression. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance.
Where molality 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.