pKa from Ka calculator
A Ka of 0.000018 gives a pKa of about 4.74, which the calculator rates as a weak acid: enter any positive Ka and get the pKa, a strength band and each substitution step for your notes. 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
- Ka
- 0.000018
- Pka
- 4.7447274949
- PKa
- 4.7447274949
- Strength
- weak
This pKa from Ka calculator converts a known acid dissociation constant into the logarithmic acidity scale every chemistry course and analytical lab uses for weak and strong acids.
What pKa and Ka mean together
Ka is the acid dissociation constant for the equilibrium HA ⇌ H+ + A−. In the concentration form used in most introductory courses, Ka equals the product of the equilibrium concentrations of hydrogen ion and conjugate base divided by the equilibrium concentration of undissociated acid. Larger Ka means a stronger acid that dissociates more completely in water; smaller Ka means a weaker acid that stays mostly protonated. pKa is the common logarithmic companion of that constant: by definition pKa equals the negative base-ten logarithm of Ka.
When Ka is one times ten to the minus five, pKa is five; when Ka is one times ten to the minus four point seven four, pKa is about four point seven four—the classic acetic-acid classroom value near one point eight times ten to the minus five for Ka. Because the scale is logarithmic, each unit change in pKa corresponds to a tenfold change in Ka, which makes comparisons between acids far easier than comparing raw scientific-notation constants.
Strong mineral acids often have negative pKa values when Ka is greater than one; very weak acids can push pKa well above seven when Ka is tiny. Those extremes remain valid under the same closed formula; what is never valid is a zero or negative Ka, because the logarithm is only defined for positive real numbers. Teachers, lab notebooks, and automated graders care about that rejection as much as the happy path: a silent fallback would hide a data-entry error and produce non-finite or meaningless results.
Practical tips, limits, and related searches
Keep Ka inside the published absolute range so minus log ten of Ka cannot overflow into a non-finite pKa the solver must reject. Use a consistent definition of Ka from your source table or experiment; if your literature quotes pKa only, invert with Ka equals ten to the power of minus pKa before other equilibrium steps rather than mixing raw Ka and pKa units inside one request.
Prefer this capability when you already know Ka and only need pKa; if your source is only measured pH and analytical concentration of a weak acid, use a dedicated Ka-from-pH capability first. Round-trip intuition is easy: ten to the power of minus the published pKa recovers the original Ka within the same twelve-decimal rounding for typical aqueous values, and confirming strength bands flip correctly when you move Ka across powers of ten that cross the classroom thresholds at pKa zero, four, and seven.
What you can do with it
Homework checks for pKa = −log10(Ka)
Students enter textbook Ka values and compare hand-written pKa and strength labels against a deterministic result before submitting.
Lab notebook conversion of tabulated Ka
Technicians convert a published or measured Ka into pKa for reports, buffer design notes, and instrument method sheets.
FAQ
How do you calculate pKa from Ka?
Use pKa = −log₁₀(Ka). This tool returns the rounded pKa, a classroom strength label, and substitution steps.
Does this calculate Ka from pH or buffer pH?
No. It only converts a known Ka into pKa. Use a dedicated Ka-from-pH or Henderson–Hasselbalch tool if you need those models.
Is the pKa from Ka calculator free?
The browser tool is free and runs locally.
Can a pKa be negative?
Yes. Strong mineral acids with a Ka greater than 1 have negative pKa values, while very weak acids with a tiny Ka can go well above 7. The same formula covers both; only a Ka of zero or below is refused.
What do the strength labels mean?
The calculator sorts each result into a classroom band: strong, moderately weak, weak or very weak. A pKa from 4 up to just under 7, such as the 4.74 of a Ka of 0.000018, counts as weak.
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-from-ka \
-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-from-ka", {
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-from-ka",
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-from-ka", 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-from-ka", 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_from_ka",
"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
Zero or negative Ka is rejected with a clear invalid_input error so a blank cell or a sign typo never silently produces a non-finite logarithm. Homework checks, buffer prep from tabulated Ka values, exam review, automated graders, and API pipelines all need that same closed formula without a spreadsheet or a full aqueous speciation package.
What pKa and Ka mean together
Related quantities such as Kb, pKb, percent ionization, and buffer pH from the Henderson–Hasselbalch equation are intentionally out of scope here so this landing stays focused on the single search intent of converting a known Ka into pKa for homework, lab prep, and API automation without a full aqueous speciation package or spreadsheet macros.
How to call the API and what comes back
Send a JSON body with one required finite number: ka (aliases Ka, acid_dissociation_constant, dissociation_constant, k_a, ka_value). Absolute value 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. Ka must be strictly positive; zero and any negative value are rejected with invalid_input. On success the response echoes rounded ka and Ka, sets pka and pKa to the same rounded value, formula to the string pKa equals minus log ten of Ka, strength to a classroom band such as strong, moderately_weak, weak, or very_weak, message to a short English summary, and steps to an ordered array of substitution lines. Failed validation—missing Ka, non-finite numbers, values outside absolute limits, zero or negative Ka, or a non-finite computed pKa—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 sending numeric JSON rather than pre-formatted strings when you control the client; string numbers are accepted only when they match a strict finite numeric pattern including optional scientific notation for values such as one point eight e minus five for acetic acid.
Practical tips, limits, and related searches
When teaching, pair classic examples: Ka equals one point eight times ten to the minus five yields pKa near four point seven four and a weak-acid band; Ka equals one times ten to the minus seven yields pKa seven and a very weak band; Ka equals ten yields pKa minus one and a strong band; and Ka equals zero demonstrates the hard invalid_input path that protects logarithms from non-positive arguments. This landing targets people searching for a pKa from Ka calculator, pKa equals minus log Ka, or convert Ka to pKa online; it is not a buffer calculator, not a percent-ionization tool, and not a Ka-from-pH solver. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when learners deliberately send zero.
Integration use cases
Unit tests for chemistry helpers
Generate known-good weak-acid pKa cases plus deliberate zero-Ka rejections for coding kata and automated grading pipelines.
Technical questions
What happens if Ka is zero or negative?
pKa is undefined for non-positive Ka. The API returns invalid_input instead of taking a log of zero or a negative number.
What pKa does acetic acid have if Ka ≈ 1.8×10⁻⁵?
pKa ≈ 4.74. Enter Ka = 1.8e-5 (or 0.000018) to see the rounded result and steps.