Weight-volume % to molarity calculator
A 5.85 % w/v sodium chloride solution, with a molar mass of 58.44 g/mol, is about 1 mol/L. Enter the label percentage and the molar mass and get the molarity, the grams per litre and each step of the arithmetic. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Weight volume percent
- 5.85
- Molar mass
- 58.44
- Molarity
- 1.001026694 mol/L
- Mass solute g per L
- 58.5
A weight-volume percent to molarity calculator turns a labeled pharmacy or laboratory concentration into the mole-per-liter figure used in kinetics, equilibria, and titration arithmetic. Enter the weight-volume percent as grams of solute per one hundred milliliters of solution and the molar mass of the solute in grams per mole; the tool returns molarity in moles per liter, the intermediate grams of solute per liter of solution, a readable formula, and step-by-step substitution so you can check the arithmetic by hand.
What weight-volume percent and molarity mean and how they convert
Weight-volume percent, often written percent w slash v or % w/v, is grams of pure solute dissolved in enough solvent to make one hundred milliliters of finished solution. It is common on stock bottles for saline, acids, and clinical reagents because the label can be written without a densitometer: five point eight five percent sodium chloride means five point eight five grams of salt per one hundred milliliters of solution.
Molarity, often written capital M, is moles of solute per liter of solution and is the concentration that appears in rate laws, equilibrium constants, and most analytical methods. Because molarity needs moles while percent weight-volume needs only mass and volume, you convert by scaling mass to a one-liter basis and dividing by molar mass. Ten times the percent weight-volume number equals grams of solute per liter of solution; dividing that mass by the solute molar mass in grams per mole yields moles per liter.
The closed formula is therefore M equals ten times percent w slash v divided by M_w. Molar mass must be strictly positive; a zero or negative value is not physical for a real pure solute and would make the division undefined or reverse the meaning of concentration. This calculator implements that conversion only: it does not invert molarity back to percent alone, convert mass-percent or volume-percent labels, look up molar masses from periodic tables, or model multi-solute mixtures with partial volumes.
Practical tips, limits, and related searches
Read the bottle carefully: percent weight-volume is not the same as percent weight-weight or percent volume-volume; using the wrong percent family will scale the mass basis incorrectly even if the arithmetic looks fine. Keep molar mass in grams per mole so the factor of ten that converts percent w slash v into grams per liter stays consistent; do not mix kilograms per mole without converting first.
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. Prefer this capability when you already have % w/v and M_w and only need M; if your source is grams and liters without a percent label, divide mass by volume first to form the percent.
What you can do with it
Homework checks for percent-to-molar conversion
Students enter % w/v and molar mass from textbook problems and compare hand-written molarity and grams-per-liter against a deterministic result before submitting.
Pharmacy stock label reconciliation
Technicians convert a weight-volume percent on a stock bottle into the molar working concentration used at the bench without opening a spreadsheet for each check.
FAQ
What does % w/v mean?
Weight-volume percent means grams of solute per 100 mL of finished solution. For example, 5% w/v means 5 g of solute in every 100 mL of solution.
Is % w/v the same as % w/w?
No. % w/v is mass per volume of solution; % w/w is mass of solute per mass of solution. They are not interchangeable without density.
Is the weight-volume percent to molarity calculator free?
The browser tool is free and runs locally.
How do I convert % w/v to molarity?
Multiply the percentage by 10 to get grams per litre, then divide by the molar mass. For 5.85 % w/v sodium chloride that is 58.5 g/L divided by 58.44 g/mol, or about 1 mol/L.
Can it convert molarity back into % w/v?
No, it works in one direction only, from % w/v to molarity. It also does not look up molar masses for you or handle mass percent or volume percent labels, which have their own calculators.
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/weight-volume-to-molarity \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"weight_volume_percent":5.85,"molar_mass":58.44}'const res = await fetch("https://api.kit.forhosting.com/chem/weight-volume-to-molarity", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"weight_volume_percent": 5.85,
"molar_mass": 58.44
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/weight-volume-to-molarity",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"weight_volume_percent": 5.85,
"molar_mass": 58.44
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/weight-volume-to-molarity", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"weight_volume_percent":5.85,"molar_mass":58.44}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"weight_volume_percent":5.85,"molar_mass":58.44}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/weight-volume-to-molarity", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"weight_volume_percent": 5.85,
"molar_mass": 58.44
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.weight_volume_to_molarity",
"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 molar mass is zero or negative the conversion is undefined or non-physical, so the API returns a clear invalid_input error instead of Infinity or a silent NaN.
How to call the API and what comes back
Weight-volume percent must be non-negative; molar mass must be strictly positive—values at or below one times ten to the minus fifteen, or any non-positive molar mass, are rejected. Send a JSON body with two required finite numbers: weight_volume_percent (aliases percent_wv, wv_percent, w_v, percent, pct, concentration_percent) as non-negative percent weight-volume, and molar_mass (aliases M_w, Mw, molecular_weight, solute_molar_mass, mm) in grams per mole. Absolute values must stay within the published limit of one trillion so intermediate products remain finite in IEEE double precision on the Worker and in the browser. On success the response echoes rounded weight_volume_percent and molar_mass, sets molarity to the rounded mol-per-liter value, unit to the string mol/L, mass_solute_g_per_L to ten times the percent, formula to M equals ten times percent w slash v over M_w, 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, negative percent, or zero or negative molar mass—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
For teaching, pair examples: five point eight five percent sodium chloride with molar mass fifty-eight point four four grams per mole yields one point zero zero one molar; ten percent glucose with molar mass one hundred eighty point one six yields about zero point five five five molar; and molar mass zero demonstrates the hard invalid_input path that protects division by a non-physical molecular weight. This landing targets people searching for a weight volume percent to molarity calculator, percent w/v to molar concentration, or convert % w/v online; it is not a mass-percent tool, not a molality converter, and not a density-based molality-to-molarity path. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when learners deliberately set molar mass to zero. Round-trip checks are easy: recompute ten times percent divided by molar mass by hand and confirm it equals the published molarity within the same twelve-decimal rounding, and confirm that mass_solute_g_per_L equals ten times the published weight_volume_percent.
Integration use cases
Unit tests for solution-chemistry software
Generate known-good % w/v to molarity cases plus deliberate zero or negative molar-mass rejections for coding kata and API contract tests.
Technical questions
How do you convert weight-volume percent to molarity?
Use M = 10 · (% w/v) / M_w, where % w/v is grams of solute per 100 mL of solution and M_w is solute molar mass in g/mol. The result M is molarity in mol/L.
What happens when molar mass is zero or negative?
The conversion is undefined or non-physical. This tool returns invalid_input instead of Infinity or NaN.