Weak base pH calculator
Solving the weak-base equilibrium for 0.1 mol/L and a Kb of 0.000018 puts the pH a little above 11: enter the concentration and Kb and get the hydroxide concentration, the pOH, the pH at 25 °C and how much base reacted. 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
- Kb
- 0.000018
- C0
- 0.1
- Unit concentration
- mol/L
- Kb
- 0.000018
- Oh minus
- 0.001332671
A weak base pH calculator turns the analytical concentration of a monoprotic base and its Kb into the equilibrium pH every general-chemistry homework set expects. Enter C in moles per litre and Kb for B plus water in equilibrium with BH-plus and OH-minus; the tool solves the quadratic Kb equals x squared over C minus x for hydroxide, then returns pOH, pH equals fourteen minus pOH at twenty-five degrees Celsius, fraction dissociated, undissociated base left, an acidity label, and step-by-step substitution you can paste into a notebook. Ammonia stock solutions, amine lab prep, exam review, and automated graders all need that same dilute weak-base path without a full aqueous speciation package.
What weak-base equilibrium and pH mean together
A monoprotic weak base B only partly accepts a proton from water. At equilibrium the species are undissociated B, conjugate acid BH-plus, and hydroxide OH-minus. With activities approximated by molar concentrations, the base dissociation constant is Kb equals the product of the conjugate acid and hydroxide concentrations divided by the free base concentration.
If the analytical concentration of base prepared is C moles per litre and x is the equilibrium hydroxide concentration, charge and mass balance for a pure solution of B give Kb equals x squared over C minus x. That is a quadratic equation in x: x squared plus Kb times x minus Kb times C equals zero. The physical root is the positive branch of the quadratic formula, x equals negative Kb plus the square root of Kb squared plus four Kb C, all over two.
From that hydroxide concentration you compute pOH as minus the base-ten logarithm of x, then pH as fourteen minus pOH when the water ion product Kw is taken as one times ten to the minus fourteen at about twenty-five degrees Celsius. Strong bases that dissociate completely are out of scope; they would set x equal to C without using Kb. Teachers and automated graders care about rejecting non-positive C or Kb as much as the happy path: a silent clamp would hide a data-entry error.
The check path recomputes the same quadratic and logs with the fixed rounding used for published fields, so a successful response always ships with steps you can assert in tests or lab notebooks.
Practical tips, limits, and related searches
Keep C and Kb inside the published absolute range so the discriminant and logarithms cannot overflow into non-finite results the solver must reject. Use moles per litre consistently; if your source reports millimoles per litre, convert before calling rather than mixing unit systems inside one request. The exact quadratic is used rather than the common square-root approximation, so cases where the degree of dissociation is not tiny still match textbook quadratic solutions.
Prefer this capability when you already know analytical C and Kb for a pure monoprotic weak base; if your source is only [OH−] already measured, use a direct pOH or pH-from-concentration path instead. Round-trip intuition is easy: recomputing Kb from published oh and undissociated should recover the input Kb within the same twelve-decimal rounding for well-behaved dilute cases, and confirming acidity stays basic for typical classroom weak bases with pH greater than seven.
What you can do with it
Homework checks for ammonia and amine pH
Students enter textbook C and Kb for weak bases and compare hand-written [OH−], pOH, and pH against a deterministic quadratic result before submitting.
Lab stock solution pH estimates
Technicians estimate equilibrium pH of dilute ammonia or organic base stocks from label molarity and tabulated Kb without a full speciation package.
FAQ
What happens if concentration or Kb is zero or negative?
Both analytical concentration and Kb must be strictly positive for the dilute weak-base model.
Does this use the √(Kb·C) approximation?
No. It uses the exact physical root of the quadratic so moderate dissociation is handled correctly.
Is this for strong bases like NaOH?
No. Strong bases that fully dissociate are out of scope; this landing is for monoprotic weak bases defined by Kb and analytical C.
Is the weak base pH calculator free?
The browser tool is free and runs locally.
What does the result include besides the pH?
You get the hydroxide and hydrogen ion concentrations, the pOH, the fraction of base that reacted with water, the base left undissociated and an acidic, neutral or basic label, plus every substitution step for your notebook.
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-base \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"concentration":0.1,"kb":0.000018}'const res = await fetch("https://api.kit.forhosting.com/chem/ph-weak-base", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"concentration": 0.1,
"kb": 0.000018
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/ph-weak-base",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"concentration": 0.1,
"kb": 0.000018
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/ph-weak-base", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"concentration":0.1,"kb":0.000018}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"concentration":0.1,"kb":0.000018}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/ph-weak-base", 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,
"kb": 0.000018
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.ph_weak_base",
"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
Zero or non-positive concentration or Kb is rejected with a clear invalid_input error so a blank cell or a sign typo never silently produces a non-finite log.
How to call the API and what comes back
Both concentration and Kb must be strictly positive; values at or below one times ten to the minus fifteen are treated as zero and rejected along with any explicit negative. Send a JSON body with two required finite numbers: concentration (aliases c0, c_initial, initial_concentration, c, molarity, base_concentration) in moles per litre, and kb (aliases Kb, base_kb, dissociation_constant, k_b, equilibrium_constant). 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 concentration and c0, kb and Kb, unit_concentration mol/L, oh_minus and oh as the equilibrium hydroxide, h_plus from Kw over hydroxide, poh and pOH, ph and pH, alpha and fraction_dissociated as x over C, undissociated as C minus x, formula strings for the equilibrium and pH relations, reaction B plus water, kw and pkw constants, acidity as acidic, neutral, or basic, message as a short English summary, and steps as an ordered array of substitution lines. Failed validation—missing fields, non-finite numbers, values outside absolute limits, zero or non-positive concentration or Kb, or a non-finite computed pH—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.
Practical tips, limits, and related searches
When teaching, pair classic examples: zero point one molar ammonia with Kb about one point eight times ten to the minus five yields a basic pH near eleven; a more dilute base with the same Kb yields a lower pH closer to seven; and concentration or Kb equal to zero demonstrates the hard invalid_input path. This landing targets people searching for a weak base pH calculator, pH from concentration and Kb, or ammonia solution pH online; it is not a buffer calculator, not a strong-base pH tool that assumes full dissociation, and not a temperature-dependent Kw 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 basic pH cases plus deliberate zero concentration or zero Kb rejections for coding kata and automated grading pipelines.
Technical questions
How is pH calculated for a weak base?
Solve Kb = x²/(C − x) for x = [OH−] with the quadratic formula, then pOH = −log10(x) and pH = 14 − pOH at 25 °C (Kw = 1×10⁻¹⁴).