Osmotic pressure calculator
A 0.1 M glucose solution at 298.15 K has an osmotic pressure of about 2.45 atm, using the default gas constant of 0.08205746 L·atm/(mol·K). Enter concentration, kelvin temperature and the van 't Hoff factor, and swap in the SI constant for pascals. Free in your browser.
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Worked example
- Molar concentration
- 0.1
- Absolute temperature
- 298.15
- Current (A)
- 1
- Concentration
- 0.1
- Molarity
- 0.1
- Creativity
- 298.15
- Temperature kelvin
- 298.15
An osmotic pressure calculator turns molar concentration, absolute temperature, and the van 't Hoff factor into the colligative pressure π equals i times c times R times T that a solution exerts across a semipermeable membrane. Concentration, the van 't Hoff factor, and the gas constant must also be positive so the product never invents a non-physical zero or negative magnitude from bad inputs. The default gas constant is 0.08205746 litre atmospheres per mole per kelvin, matching textbook chemistry problems that report π in atmospheres when concentration is molarity in moles per litre; pass SI R of about 8.314462618 joules per mole per kelvin with concentration in moles per cubic metre when you need pascals.
What osmotic pressure and the van 't Hoff equation mean
Osmotic pressure is the colligative property that quantifies how strongly a solution draws pure solvent across a semipermeable membrane that blocks solute particles. In introductory physical chemistry the ideal dilute limit is the van 't Hoff equation: pi equals i times c times R times T, where c is molar concentration of solute particles as prepared, T is absolute temperature, R is the gas constant in units that make pressure come out consistently, and i is the van 't Hoff factor accounting for dissociation or association.
For a non-electrolyte such as sucrose or glucose, i is one because each formula unit contributes one dissolved particle. For strong electrolytes, i approaches the number of ions produced per formula unit under ideal complete dissociation—about two for NaCl, about three for CaCl2—though real solutions often show effective i values slightly below those integers because of ion pairing.
Units of concentration and R must match each other: with the default R of about 0.082057 litre atmospheres per mole per kelvin, concentration is molarity in moles per litre and pi is in atmospheres; if you need SI pressure in pascals, supply R about 8.314462618 joules per mole per kelvin and express concentration in moles per cubic metre.
The check path recomputes i times c times R times T with the same fixed rounding used for published fields, so a successful response always ships with steps you can assert in tests, lab reports, or chemistry worksheets about colligative properties.
Practical tips, limits, and related chemistry searches
Convert Celsius to kelvin before calling the tool, and never pass a temperature of zero or below on the absolute scale. Keep inputs inside the published absolute range so the product i c R T cannot overflow into a non-finite result that 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.
Prefer this capability when you already have molar concentration, absolute temperature, and an estimated i and only need pi under the ideal van 't Hoff assumption; if your solution is concentrated or highly non-ideal, adjust i or use a more advanced activity model.
What you can do with it
Chemistry homework checks for π = iCRT
Students enter molarity, kelvin temperature, and van 't Hoff factor from textbook colligative-property problems and compare hand-written π against a deterministic result before submitting.
Lab prep for dilute solution estimates
Quickly estimate osmotic pressure for a prepared molarity and bath temperature when the ideal van 't Hoff model is acceptable for bench work.
FAQ
What is the osmotic pressure formula?
The van 't Hoff equation is π = i·c·R·T: van 't Hoff factor times molar concentration times the gas constant times absolute temperature in kelvin.
What van 't Hoff factor i should I use?
Ideal complete dissociation gives about 2 for NaCl and about 3 for CaCl₂; real solutions may use a slightly lower effective i.
What gas constant R should I use?
Default R is 0.08205746 L·atm/(mol·K) so π is in atmospheres when c is mol/L. For SI (Pa) with c in mol/m³, pass about 8.314462618 J/(mol·K) as r.
Is the osmotic pressure calculator free?
The browser tool is free and runs locally.
Why is the measured osmotic pressure of a salt solution often a little lower?
Because real strong electrolytes rarely dissociate perfectly. Ion pairing makes the effective van 't Hoff factor slightly smaller than the ideal 2 for sodium chloride or 3 for calcium chloride, so the measured pressure falls a little below the ideal value.
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/osmotic-pressure \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"c":0.1,"t":298.15,"i":1}'const res = await fetch("https://api.kit.forhosting.com/chem/osmotic-pressure", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"c": 0.1,
"t": 298.15,
"i": 1
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/osmotic-pressure",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"c": 0.1,
"t": 298.15,
"i": 1
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/osmotic-pressure", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"c":0.1,"t":298.15,"i":1}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"c":0.1,"t":298.15,"i":1}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/osmotic-pressure", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"c": 0.1,
"t": 298.15,
"i": 1
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.osmotic_pressure",
"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
Temperature must be strictly positive on the kelvin scale: zero or negative kelvin is rejected with a clear invalid_input error because absolute temperature cannot be non-positive in this model.
What osmotic pressure and the van 't Hoff equation mean
Absolute temperature must be in kelvin (or another absolute scale consistent with R); converting from Celsius means adding two hundred seventy-three point one five before you call the API, because a non-positive kelvin value has no meaning for this equation and is rejected outright. Teachers and automated graders care about the non-positive temperature rejection as much as the success path: a learner who forgets the Celsius-to-kelvin shift should see an explicit invalid_input message instead of a nonsense negative absolute temperature.
How to call the API and what comes back
Concentration, van 't Hoff factor, and gas constant must be strictly positive; temperature in particular must be positive kelvin. Send a JSON body with three required finite numbers: c (aliases concentration, molarity, M, and molar), t (aliases temperature, T, temp, and kelvin), and i (aliases van_t_hoff, vant_hoff, vanthoff, factor, and i_factor). Optionally include r (aliases R, gas_constant, and gasConstant); when omitted the solver uses 0.08205746. 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 c, t, i, and r, mirrors them as concentration, molarity, temperature, temperature_kelvin, van_t_hoff, factor, and gas_constant, sets pi, osmotic_pressure, and pressure to the same rounded osmotic pressure, formula to the string pi equals i times c times R times T, equation to a filled substitution string, message to a short English summary, and steps to an ordered array of calculation lines. Failed validation—missing fields, non-finite numbers, values outside absolute limits, non-positive concentration or factor or R, or non-positive temperature in kelvin—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 chemistry searches
When teaching chemistry, pair examples: one-tenth molar glucose at two hundred ninety-eight point fifteen kelvin with i equals one and default R recovers about two point four five atmospheres; half-molar ideal sodium chloride at the same temperature with i equals two doubles that scale; use SI R and mol per cubic metre when you need pascals for lab instruments; and deliberately send t equals zero or a negative kelvin value to demonstrate the hard invalid_input path that guards against forgotten unit conversions. This landing targets people searching for an osmotic pressure calculator, van 't Hoff factor colligative properties tool, or pi equals iCRT homework help online; it is not a reverse solver for concentration from measured pressure, not a membrane flux simulator, and not a standalone Celsius-to-kelvin converter. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when learners deliberately send bad temperatures. Round-trip checks are easy: recompute i times c times R times T by hand and confirm it equals the published osmotic_pressure within the same twelve-decimal rounding.
Integration use cases
Unit tests for student osmotic solvers
Generate known-good π values for non-electrolytes and electrolytes plus deliberate non-positive kelvin rejections for coding kata expected values in chemistry tools.
Technical questions
Why must temperature be in kelvin?
Osmotic pressure uses absolute temperature. Zero or negative kelvin is not physically valid for this model; this tool returns invalid_input for non-positive temperature.