Molality calculator
0.5 mol of solute in 1 kg of solvent is 0.5 mol/kg, and every substitution step is written out next to the answer. Enter the solvent mass in kilograms, not grams, or the result is off by a factor of 1000. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Moles
- 0.5
- Solvent mass kg
- 1
- Molality
- 0.5 mol/kg
A molality calculator built on the textbook definition turns two laboratory quantities—moles of solute and the mass of the solvent in kilograms—into the concentration chemists write as m, moles of solute per kilogram of solvent. Homework checks, exam prep, colligative-property pre-work, solution prep logs, and coding kata expected values all need that same deterministic map without opening a spreadsheet.
What molality means and why mass is in kilograms
Molality m is defined as the amount of solute in moles divided by the mass of solvent alone, expressed in kilograms. That definition is temperature-independent in a way molarity is not: heating a solution changes volume and therefore moles per litre, but it does not change how many moles of solute you dissolved into a given mass of solvent. Students often confuse molality with molarity because both use the letter m or M and both involve moles of solute; the difference is the denominator—kilograms of solvent versus litres of solution.
Another common trap is leaving solvent mass in grams: if you divide moles by five hundred grams without converting to zero point five kilograms, the numerical result is off by a factor of one thousand. This calculator expects solvent mass already in kilograms so the formula stays the pure definition with no hidden unit conversion that would surprise someone pasting lab-notebook values. Zero moles of solute in any positive solvent mass yields molality zero, which is physically pure solvent and a valid homework edge case.
Practical tips, limits, and related searches
Convert solvent mass to kilograms before you call: five hundred grams of water is zero point five kilograms, not five hundred, or the reported molality will be wrong by three orders of magnitude. Prefer sending numbers rather than pre-formatted strings when you control the client. Near-zero solvent masses within one times ten to the minus fifteen of zero are treated as zero and rejected so floating noise cannot invent a huge molality.
Prefer this capability when you already have n and solvent mass in kilograms and only need m; if your source lists mass percent, mole fraction, or molarity, convert those quantities first or use a dedicated tool.
What you can do with it
Homework checks for the molality definition
Students enter moles of solute and solvent mass in kg from textbook problems and compare hand-written m and substitution steps against a deterministic result before submitting.
Pre-flight before colligative-property solvers
Pipelines compute molality from lab moles and solvent mass first, then feed m into freezing-point depression or boiling-point elevation modules that expect molal concentration.
FAQ
What happens when solvent mass is zero?
Molality is undefined (division by zero). Negative solvent mass is also rejected.
Can I pass solvent mass in grams?
No. Convert to kilograms first (divide grams by 1000).
Is zero moles of solute allowed?
Yes. Zero moles in any positive solvent mass yields molality zero (pure solvent). Negative moles are rejected.
Is the molality calculator free?
The browser tool is free and runs locally.
What is the difference between molality and molarity?
The denominator. Molality divides moles of solute by kilograms of solvent, while molarity divides by litres of the whole solution. Because heating changes volume but not mass, only molarity shifts with temperature.
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-from-moles-mass \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"moles":0.5,"solvent_mass_kg":1}'const res = await fetch("https://api.kit.forhosting.com/chem/molality-from-moles-mass", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"moles": 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-from-moles-mass",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"moles": 0.5,
"solvent_mass_kg": 1
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/molality-from-moles-mass", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"moles":0.5,"solvent_mass_kg":1}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"moles":0.5,"solvent_mass_kg":1}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/molality-from-moles-mass", 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": 0.5,
"solvent_mass_kg": 1
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.molality_from_moles_mass",
"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 solvent mass is zero or negative the division is undefined or non-physical, so the API returns a clear invalid_input error instead of infinity or a silent NaN.
What molality means and why mass is in kilograms
Textbooks write m equals n over mass_solvent with mass_solvent in kilograms, so a one-molal aqueous solution means one mole of solute per kilogram of water, not per litre of solution. Zero solvent mass is not valid: there is no solvent to define a per-kilogram concentration, and the API refuses that input rather than returning infinity.
How to call the API and what comes back
Moles must be non-negative; solvent mass must be strictly positive in kilograms. Send a JSON body with two required finite numbers: moles (aliases n, n_solute, moles_solute, solute_moles, and amount) and solvent_mass_kg (aliases mass_solvent_kg, kg_solvent, solvent_mass, mass_kg, and w_solvent). Absolute values must stay within the published limit of one trillion so intermediate results remain finite in IEEE double precision on the Worker and in the browser. On success the response echoes rounded moles and solvent_mass_kg, sets molality to the rounded quotient, unit to the string mol/kg, formula to the closed definition string m equals n over mass_solvent_kg, message to a short English summary naming the inputs and result, and steps to an ordered array of substitution lines you can display or assert. Failed validation—missing fields, non-finite numbers, values outside absolute limits, negative moles, zero or negative solvent mass, or a non-finite computed molality—returns invalid_input and is never charged. Pricing is a flat $0.002 per successful request with unit item and zero marginal unit price. Prefer numeric JSON values when you control the client; string numbers are accepted only when they match a strict finite numeric pattern, which keeps accidental unit labels and comma-separated text from looking like successful input.
Practical tips, limits, and related searches
When teaching, pair examples: zero point five moles in one kilogram of solvent yields zero point five molal with transparent steps; two moles in zero point five kilograms yields four molal; zero moles in any positive mass yields zero; and solvent_mass_kg equals zero demonstrates the hard invalid_input path that matches the product promise. This landing targets people searching for a molality calculator from moles and solvent mass, m equals n over kg, or moles of solute per kilogram of solvent online; it is not a mole-fraction-to-molality converter that needs solvent molar mass, not a molality-to-molarity converter that needs density, and not a colligative-property boiling-point calculator. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when learners deliberately set solvent mass to zero. Round-trip checks are easy: recompute moles divided by solvent mass by hand and confirm it equals the published molality within the same twelve-decimal rounding, and confirm that molality times solvent_mass_kg recovers the original moles.
Integration use cases
Unit tests for chemistry tooling
Generate known-good n and kg pairs plus deliberate zero or negative solvent-mass rejections for coding kata expected values.
Technical questions
What is the formula for molality?
Molality m equals moles of solute n divided by the mass of solvent in kilograms: m = n / mass_solvent_kg. The unit is mol/kg (often written m).