ForHosting KIT · Science

Ka from pH calculator

Turn a measured pH into the acid dissociation constant: a 0.1 mol/L weak acid at pH 3 holds 0.001 mol/L of hydrogen ions, which gives a Ka of 0.00001010101 and a pKa of 4.995635194598. Free in your browser.

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Worked example

If you enter

Analytical: 0.1
Ph: 3

You get

H plus: 0.001
Ha eq: 0.099
Ka: 0.000010101
Pka: 4.9956351946

The acid dissociation constant Ka quantifies how far a monoprotic weak acid HA splits into hydrogen ions and conjugate base at equilibrium. When you know the analytical concentration of the acid and the measured pH of that pure solution, the standard ICE table gives a direct route: hydrogen ion concentration is ten to the minus pH, and Ka equals that squared value divided by the remaining undissociated acid concentration. This capability applies that textbook formula deterministically, returns Ka together with pKa and percent dissociation, and rejects a zero concentration with a clear invalid input error so homework checkers never see NaN or infinity.

From a pH reading to Ka

A weak acid such as HA only partly splits into hydrogen ions and its conjugate base. If you know how much acid you dissolved and you measure the pH of that solution, the equilibrium is fixed. The hydrogen ion concentration is ten to the minus pH, the conjugate base matches it, and the acid left intact is the starting concentration minus that amount. Ka is the hydrogen ion concentration squared, divided by the acid that remains. For 0.1 mol/L at pH 3 that means 0.001 mol/L of hydrogen ions, 0.099 mol/L of intact acid, a Ka of 0.00001010101 and 1 % dissociation.

When the result holds, and when it is refused

The method assumes a pure solution of one monoprotic weak acid in water, with the water's own ionization small next to the acid's. A concentration of zero is refused, because Ka has no meaning without any acid. So is a pH so low that ten to the minus pH reaches or passes the concentration, since no intact acid would then be left, which is not physical in this model. Alongside Ka you also get pKa and the percent dissociation, so one pH reading answers all three questions in a homework set or a lab report.

What you can do with it

Homework and exam verification

Students compute Ka from analytical C and measured pH for monoprotic weak acids, compare against answer keys, and get a clear error when concentration is zero.

Lab notebook and report checks

Instructors and lab software turn measured pH of a known-concentration weak acid into Ka, pKa, and percent dissociation for reports and quizzes.

FAQ

Is the Ka from pH calculator free?

The browser tool runs free on your device.

How is this different from the pH to pOH converter?

This tool finds Ka (and pKa) from acid concentration and measured pH.

What happens if the concentration is zero?

The calculation is refused with a clear message, because Ka is undefined when there is no acid in the solution. Enter the analytical concentration in mol/L as a number greater than zero.

What if the pH is too low for the concentration I entered?

If ten to the minus pH reaches or exceeds the acid concentration, no undissociated acid would remain and Ka would be non-physical. The calculator refuses that case instead of returning a meaningless constant.

Does it take the ionization of water into account?

No. It treats the measured pH as coming entirely from the acid, the usual textbook assumption when water's own ionization is small next to the acid's contribution.

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

POSThttps://api.kit.forhosting.com/chem/ka-from-ph

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/ka-from-ph \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"concentration":0.1,"ph":2.87}'

const res = await fetch("https://api.kit.forhosting.com/chem/ka-from-ph", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "concentration": 0.1,
    "ph": 2.87
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/chem/ka-from-ph",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "concentration": 0.1,
    "ph": 2.87
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/ka-from-ph", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"concentration":0.1,"ph":2.87}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"concentration":0.1,"ph":2.87}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/ka-from-ph", 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,
  "ph": 2.87
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "chem.ka_from_ph",
  "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

Per request$0.002

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, lab instructors, and tutoring pipelines can pin the JSON in tests without installing a chemistry CAS.

What Ka from pH means and which inputs to send

For a monoprotic weak acid HA in water at analytical concentration C moles per liter, the equilibrium is written HA reverse arrows H plus plus A minus. When the measured pH comes from that pure acid solution and water autoionization is neglected relative to the acid contribution, the ICE table sets hydrogen ion concentration equal to ten raised to the minus pH, conjugate base equal to the same value, and undissociated acid equal to C minus hydrogen ion concentration. The acid dissociation constant is then Ka equals hydrogen ion concentration squared divided by open parenthesis C minus hydrogen ion concentration close parenthesis. Negative concentration is rejected. Send the required field concentration as a finite number strictly greater than zero in mol per liter—aliases include c, C, conc, molarity, and acid_concentration. Send the required field ph as a finite measured pH—aliases include pH and p_h. Concentration equal to zero is invalid_input because Ka is undefined when there is no analytical acid amount. If ten to the minus pH is greater than or equal to C, the pure monoprotic ICE denominator is not positive, so the engine also returns invalid_input rather than a nonphysical Ka. Identical inputs always produce identical JSON because the path is pure arithmetic with no network, no random seed, and no wall clock. This landing targets the search intent to find Ka from concentration and pH, which is distinct from converting pH to pOH alone or computing degree of dissociation from two concentrations.

Reading the response, special cases, and classroom checks

A classic classroom check is C equals zero point one mol per liter with pH equals three: hydrogen ion is zero point zero zero one, Ka is about one point zero one times ten to the minus five, and percent dissociation is one percent. On success the response echoes concentration and ph after validation, then returns h_plus equal to ten to the minus pH, ha_eq equal to C minus h_plus, ka with stable twelve-decimal intermediate rounding, pka equal to negative log base ten of Ka, and percent_dissociation equal to one hundred times h_plus over C. The unit_ka field is always mol per liter, the formula string restates Ka equals open parenthesis H plus close parenthesis squared over open parenthesis C minus H plus close parenthesis with H plus from pH, and method is always ka_from_ph so pipelines can assert the algorithm without sniffing shapes. Missing fields, non-numeric strings, zero or negative concentration, and hydrogen ion not strictly below C all fail with invalid_input and are not charged. Downstream pH-to-pOH tools answer a different question; this endpoint stops at Ka and pKa from concentration and pH so each SEO intent stays focused.

API usage, pricing, lab workflows, and practical limits

Call the free browser tool for lecture demos, lab notebook checks, and quick homework verification without installing a chemistry computer-algebra system. Keep concentration in mol per liter and pH as a finite measured value inside a practical range; the engine will not invent activity coefficients, polyprotic stepwise constants, ionic strength corrections, or temperature dependence of Ka. Authors shipping SDKs can map ka and pka straight into UI fields labeled Ka and pKa for students who already know those symbols from textbooks. Send a JSON body with required numeric fields concentration and ph, optionally using the documented aliases if your schema already names c and pH. Use the prepaid API when a tutoring platform, LIMS helper, exam-autograder sandbox, or equilibrium pipeline 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 a pH to pOH converter when you already have pH and only need hydroxide concentration, or with degree-of-dissociation helpers when you already know initial and equilibrium undissociated concentrations rather than pH.

Integration use cases

Tutoring and autograder pipelines

Assessment sandboxes call a stable prepaid endpoint for exact Ka = [H+]^2/(C−[H+]) instead of reimplementing the ICE formula in every language.

Technical questions

How do I calculate Ka from pH with this tool?

Send JSON with concentration (C > 0 mol/L) and ph (measured pH). The response returns ka = [H+]^2/(C−[H+]) with [H+] = 10^(−pH), plus pka and percent_dissociation.