pKa calculator
A Ka of 1 × 10⁻⁷ is exactly pKa 7, and acetic acid's 1.8 × 10⁻⁵ is about 4.74: enter a positive Ka, typed plainly or in scientific notation, and get its pKa for tables, homework and drills. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Ka
- 0.000018
- Pka
- 4.7447274949
The acid dissociation constant Ka tells how far a weak acid HA splits into hydrogen ion and conjugate base at equilibrium. Chemists usually quote that strength on a compact logarithmic scale called pKa, defined as the negative base-ten logarithm of Ka. This capability accepts one positive finite Ka and returns the matching pKa with a pure deterministic formula: no network, no temperature tables, and no random behaviour. Ka that is missing, non-numeric, zero, or negative is rejected with a clear invalid input error because the real logarithm is undefined and would never describe a physical equilibrium constant.
What Ka and pKa mean and when this conversion applies
The acid dissociation constant Ka is the equilibrium constant of HA ⇌ H⁺ + A⁻, or equivalently HA plus water producing hydronium and the conjugate base. It is always a positive number in a well-posed aqueous equilibrium: larger Ka means a stronger acid and a higher equilibrium hydrogen-ion concentration at a given analytical concentration. Reporting Ka in scientific notation is clumsy in tables, so chemists fold it onto a compact logarithmic scale called pKa, defined as the negative base-ten logarithm of Ka.
Each unit of pKa is a ten-fold change in Ka, and a smaller pKa is a stronger acid. Acetic acid with Ka near 1.8×10⁻⁵ therefore has pKa near 4.74; a Ka of 1×10⁻⁷ has pKa exactly 7; a Ka of 1 has pKa of 0. This calculator performs that single conversion and nothing else.
It does not estimate the pH of a weak-acid solution from concentration, does not apply the Henderson–Hasselbalch buffer equation, and does not convert pKa into pKb. Those are different search intents served by other landings. Use this tool when the question is specifically pKa from Ka rather than a full acid–base package.
What you can do with it
Lab notebook and LIMS helpers
Bench apps convert recorded Ka into pKa fields for sample records without re-deriving the log formula in every language.
FAQ
Does the pKa calculator cost anything in the browser?
The browser tool runs free on your device.
Can I send pH or concentration instead of Ka?
No. This product starts from Ka and returns only pKa. For pH estimates, Ka from pH, or conjugate-pair conversions, use a dedicated acid–base tool.
Does a smaller pKa mean a stronger acid?
Yes. A stronger acid splits further into ions and has a larger Ka, and because pKa is the negative logarithm, a larger Ka becomes a smaller pKa. Each unit of pKa stands for a tenfold change in Ka.
Why is a Ka of zero or below rejected?
Because the logarithm of zero or of a negative number is not defined, and such a value never describes a real equilibrium constant. The calculator flags the entry so you can fix a blank cell or a sign mistake.
What pKa do round Ka values give?
A Ka of 1 gives a pKa of 0, a Ka of 1 × 10⁻³ gives 3 and a Ka of 1 × 10⁻⁷ gives exactly 7. These whole numbers make handy checkpoints when you mark homework or test a spreadsheet.
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/pka \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"ka":0.000018}'const res = await fetch("https://api.kit.forhosting.com/chem/pka", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"ka": 0.000018
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/pka",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"ka": 0.000018
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/pka", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"ka":0.000018}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"ka":0.000018}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/pka", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"ka": 0.000018
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.pka",
"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
Students checking weak-acid homework, tutors building auto-graded drills, and pipelines that already store tabulated Ka can pin the JSON in tests.
What Ka and pKa mean and when this conversion applies
Provide the required field ka as a finite JSON number or a numeric string. Aliases Ka, K_a, and acid_dissociation_constant are accepted so notebook exports and legacy payloads do not need a rewrite. When the field is missing, empty, non-numeric, zero, or negative, the task fails with invalid_input that names ka, because a real logarithm of a non-positive Ka is undefined and would never describe a physical equilibrium constant. The algorithm is pure arithmetic: the same ka always yields the same JSON, with no wall clock, no random seed, and no remote chemistry table.
Reading the response fields and the pKa formula
For the classic weak-acid example with ka equal to 1.8×10⁻⁵ you get pka near 4.74. For ka equal to 1×10⁻³ you get pka equal to 3. For ka equal to 1 you get pka equal to 0, the log-scale origin that marks Ka of unity. Prefer the structured pka number for pipelines and auto-graders; use formula when a human-readable caption is enough for a report or a slide. The echoed ka is the numeric value after parsing, so a string such as "1.8e-5" comes back as the number 0.000018. That echo lets a caller confirm unitless Ka was interpreted as intended before storing pka beside it. On success the response echoes the parsed ka, reports pka as the negative base-ten logarithm of that value, and restates the identity in the formula string as pKa equals minus log base ten of Ka. The method label is always pka_from_ka so downstream code can branch without sniffing shapes. Validation failures for zero or negative Ka never invent a logarithm—they surface invalid_input with a message that names the field and the expectation that Ka must be a positive number. Extremely small or extremely large Ka values that overflow to a non-finite logarithm are also rejected rather than serialized as Infinity, which JSON clients would mishandle. Pair the JSON with your own ICE-table or titration code if the next step is pH; this endpoint stops at the pKa conversion and does not emit worked concentration steps.
API usage, pricing, classroom workflows, and practical limits
Call the free in-browser pKa calculator for homework checks, lecture demos, and quick lab-bench verification without installing spreadsheet macros or a local chemistry package. The model does not adjust Kw for other temperatures, does not solve ICE tables, and does not accept pH or ion concentrations as primary inputs—those are different search intents and different products. Authors shipping SDKs can map pka directly into UI tables that show Ka beside pKa—the same layout students write by hand when they convert dissociation constants on paper. Repeatable outputs make this a good oracle for exam keys that ask what the pKa of a given Ka is, and for lab systems that store Ka from a fit and need pKa on the same row without re-deriving the log in every language. Send a JSON body with the required positive finite number ka. Use the prepaid API when a tutoring platform, LMS auto-grader, curriculum generator, or lab notebook service needs a stable contract under KIT balance at $0.002 per successful item; validation failures never consume balance after the engine rejects the request. Pair this endpoint with pKb or conjugate-ratio tools when you work acid–base pairs, or with weak-acid pH calculators when the next step is an approximate hydrogen-ion estimate from concentration and Ka. Typical Ka values from tables (10⁻² down to 10⁻¹²) stay well inside the finite-logarithm range; if you paste a formatted string with units or commentary, strip it to a bare number first, because non-numeric text is invalid_input.
Integration use cases
Homework and exam verification
Students enter a tabulated Ka and check pKa against answer keys; zero or negative Ka returns a clear invalid_input error instead of a silent wrong number.
Tutoring and curriculum generators
Lesson builders produce worked Ka → pKa examples with structured JSON for flashcards, step sheets, and auto-graded drills.
Technical questions
How do I convert Ka to pKa with this calculator?
Send JSON with ka set to a positive finite number (for example ka: 1.8e-5). The response includes pka equal to −log10(Ka).
What error do I get if Ka is not a positive number?
The request fails with invalid_input. pKa = −log10(Ka) is only defined for positive Ka; missing, non-numeric, zero, and negative values are rejected and not charged.
Which formula is used for pKa?
It uses pKa = −log10(Ka). The formula string is always returned in the JSON response for display and auditing.