Weak acid pH calculator
A weak acid at 0.1 mol/L with a Ka of 0.000018 has a pH just under 3: enter the concentration and Ka or pKa and get the exact pH next to the square-root shortcut, with a note on whether the shortcut holds. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Concentration
- 0.1
- Ka
- 0.000018
- Pka
- 4.7447274949
- Ka source
- ka
- H concentration
- 0.001332671
- Ph
- 2.8752770616
A weak acid does not fully dissociate in water, so you cannot read its pH from concentration alone the way you do for a strong acid. You need the acid dissociation constant Ka, or its common log form pKa, together with the analytical concentration of HA. This calculator solves the monoprotic equilibrium with the standard quadratic formula for hydrogen-ion concentration, returns pH as minus log base ten of that root, and also reports the familiar square-root approximation so you can see when the shortcut is safe. Negative concentration is rejected with a clear validation error.
How weak-acid pH is calculated from C and Ka
Start with a monoprotic weak acid HA at analytical concentration C in moles per litre and a positive dissociation constant Ka. The equilibrium is HA in equilibrium with H plus and A minus, with Ka equal to the product of those ion concentrations divided by the undissociated acid. Charge and mass balance with the usual approximation that ignores water autoionization set x equal to the hydrogen-ion concentration and to the conjugate-base concentration, while the remaining HA is C minus x.
Substituting into the Ka expression produces the quadratic equation x squared plus Ka times x minus Ka times C equals zero. The physical non-negative root is x equals negative Ka plus the square root of Ka squared plus four Ka C, all over two. The pH is then minus the base-ten logarithm of x.
When only pKa is known, convert first with Ka equals ten to the power of negative pKa; when both Ka and pKa are supplied they must agree within floating-point tolerance. The classical square-root shortcut x approximately equal to the square root of Ka times C is also evaluated and flagged as valid when the ratio C over Ka is at least one hundred, which is the usual classroom rule of thumb that ionization remains small. Percent ionization is one hundred times x over C.
Concentration must be strictly positive; a negative molarity is rejected because it is non-physical. Zero concentration is rejected because this acid-only model would give zero hydrogen ions and an undefined pH. Non-positive Ka values are likewise rejected so the equilibrium constant stays meaningful.
Where weak-acid pH calculations show up in real work
General chemistry courses constantly assign weak-acid pH problems that require either the square-root approximation or the full quadratic, and a single trusted oracle that returns both speeds grading and demo setup without silent calculator mode mistakes. Laboratory technicians preparing stock solutions of acetic, citric mono-step models, or dilute organic acids need a fast check that the planned molarity and published Ka produce a pH in the expected band before titration day.
Environmental and water-quality notes often quote weak organic acids in natural waters and need a transparent C-and-Ka path rather than a black-box meter reading alone when teaching the underlying equilibrium. Process engineers screening dilute acid streams in food, pharma, or cleaning formulations use the same monoprotic approximation as a first-cut estimate before activity-coefficient or multiprotic models enter the spreadsheet. If you later need pOH from hydroxide concentration alone, strong-acid pH, buffer Henderson–Hasselbalch, or base Kb problems, treat those as separate landings rather than overloading this weak-acid calculator page.
The SEO slug targets people searching for a weak acid pH calculator by concentration and Ka or pKa, which is a different intent from a pure pOH converter.
What you can do with it
Homework and exam answer keys
Compute quadratic pH, percent ionization, and the √(Ka·C) check for textbook weak-acid sets so teachers and auto-graders share one ground truth.
Lab stock solution checks
Verify planned molarity and published Ka or pKa for acetic or formic acid stocks before titration or buffer prep.
FAQ
How is weak-acid pH calculated?
The square-root approximation √(Ka·C) is also reported when C/Ka ≥ 100.
Can I enter pKa instead of Ka?
Yes. Provide pka and Ka is computed as 10^(−pKa). You may supply both if they agree within floating-point tolerance.
What happens with a negative concentration?
Acid molarity cannot be negative; zero is also rejected because this model would not define a finite pH.
Does this work for strong acids or buffers?
No. Strong acids fully dissociate (different formula), and buffers need Henderson–Hasselbalch with both acid and conjugate base. This tool is monoprotic weak acid only.
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/ph-weak-acid \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"concentration":0.1}'const res = await fetch("https://api.kit.forhosting.com/chem/ph-weak-acid", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"concentration": 0.1
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/ph-weak-acid",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"concentration": 0.1
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/ph-weak-acid", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"concentration":0.1}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"concentration":0.1}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/ph-weak-acid", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"concentration": 0.1
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.ph_weak_acid",
"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
Use it for homework checks, lab prep of acetic, formic, or benzoic acid solutions, and quick API batching when you already know C and Ka.
How weak-acid pH is calculated from C and Ka
The pipeline is pure 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.
What you send and what you get back
Send concentration as a finite positive number under the field concentration, representing moles of HA per litre. Provide either ka or pka, or both if they are consistent. An acidity label classifies the computed pH as acidic, neutral, or basic relative to seven. Classic checks include acetic acid near zero point one molar with Ka about one point eight times ten to the minus five, which yields a pH near two point nine, and the same case supplied only via pKa near four point seven four. Missing concentration, a negative concentration, missing both Ka and pKa, non-numeric strings, NaN, and Infinity fail with invalid_input and are not charged. On success the response echoes cleaned concentration, ka, pka, and ka_source indicating whether the constant came from ka, pka, or both. The primary equilibrium results are h_concentration, ph, and percent_ionization from the quadratic root. For comparison you also receive h_approx and ph_approx from the square-root formula, the dimensionless ratio c_over_ka, and approximation_valid as a boolean. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance. This capability does not solve buffers, polyprotic stepwise dissociation, strong-acid complete ionization, or bases; those are separate search intents and landings.
Where weak-acid pH calculations 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.
Integration use cases
Exam generators and LMS plugins
Batch identical C-and-Ka fixtures through the API at $0.002 per item for deterministic answer keys without external chemistry services.