Chemistry calculator

Weak-Acid Equilibrium Calculator

Solve monoprotic weak-acid ICE from formal C and Ka or pKa. Charge and mass balance; optional water at 25 °C. Runs locally. Not Henderson–Hasselbalch and not pH = −log10[H+] identities.

Instant result

Result

—

Enter values to calculate.

Inputs—

Mode—

Formula—

Trust summary Engine tested · Source checked · 12/12 tests · Production surface contract 4/4 · v1.0.0

Input interpretation: Enter values to calculate.
Result: —
Model: Monoprotic weak-acid ICE pH and species from formal C and Ka or pKa.
Scope: Monoprotic HA ⇌ H+ + A−
Verification: Engine tested · 12/12 tests · Production surface contract 4/4 · Source checked · v1.0.0
Named expert review: Optional · Not performed
Sources: OpenStax Chemistry 2e — Weak Acids — Equilibrium of a weak acid
IUPAC Gold Book — acid dissociation constant — acidity constant
Sources
Evidence: 2 golden · 5 boundary · 5 property · Production surface contract 4/4 · Artifact integrity PASS
Production: Embedded snapshot: STALE · Last attested schema matched 1.0.0 snapshot / local build · Semantic contract ✓ · Last attestation PASS · current evidence changed · re-attestation required · Public/cache ✓ · Origin ✓ · Live production status PASS (0 stale; 163 CURRENT) @ 2026-09-17T03:09:03.307Z
Semantic contract: PASS

Formulas

Core equations used by this calculator.

ICE with water[H⁺]³ + Ka[H⁺]² − (Kw + Ka C)[H⁺] − Ka Kw = 0

ICE neglecting waterKa = x² / (C − x)

iKw = 1.0×10⁻¹⁴ at 25 °C. This page is not Henderson–Hasselbalch and not the water-ion identity pH = −log10[H+].

How to use

Enter formal concentration C

C is the analytical HA concentration in mol/L. It must be greater than zero.

Enter pKa or Ka

Give one acidity encoding. Sending both is rejected.

Choose whether to include water

ICE (default) includes Kw at 25 °C. ICE, no water uses the quadratic approximation.

Read pH and the ICE species

The solver reuses math.numerical.root. Outputs [H+], [HA], [A−], [OH−], and α = [A−]/C.

Example calculations

Common configurations with formula and result.

Acetic acid, water neglected

C=0.1 mol/L, Ka=1.8e-5

x = (−Ka + √(Ka² + 4 Ka C)) / 2

[H+]≈1.333e-3, pH≈2.875

From pKa

C=0.1 mol/L, pKa=5

Ka = 10^(−pKa)

Ka=1e-5; ICE pH from the cubic or quadratic

Weak-Acid Equilibrium calculator specification

Version 1.0.0 · Engine tested

Calculation status

Engine tested 12/12 tests · Production surface contract 4/4
Named expert review Not performed
Calculation version 1.0.0

Review policy · Evidence

Definition: Monoprotic HA ⇌ H+ + A−. With water: [H+]³ + Ka[H+]² − (Kw + Ka C)[H+] − Ka Kw = 0. Without water: Ka = x²/(C−x).
What it calculates: Monoprotic weak-acid ICE pH and species from formal C and Ka or pKa.
Inputs: mode?
C | c | concentration
pKa | Ka
Outputs: pH
h
HA
A
oh
C
Ka
pKa
fraction_dissociated
Formula: [H+]³ + Ka[H+]² − (Kw + Ka C)[H+] − Ka Kw = 0 (mode=ice)
Assumptions: Monoprotic HA ⇌ H+ + A−
Activity coefficients ≈ 1
Kw = 1e-14 at 25 °C when mode=ice
Not Henderson–Hasselbalch
Not pH = −log10[H+] identities
Not polyprotic, not a weak base, not titration
Units: mol/L
pH
pKa
Boundary conditions: missing C → MISSING_REQUIRED_INPUT
C ≤ 0 or Ka ≤ 0 → VALUE_MUST_BE_POSITIVE
pKa and Ka together → MIXED_INPUT_ENCODING
hh_ph is not a mode on this page
Example: C=0.1 Ka=1.8e-5 mode=ice_no_water → x=(−Ka+√(Ka²+4 Ka C))/2
Validation cases: 3 published on this page · 12/12 tests · Production surface contract 4/4 · View evidence
C=0.1 Ka=1.8e-5 mode=ice_no_water → h = (−Ka+√(Ka²+4 Ka C))/2
C=0.1 pKa=5 → pKa=5, model=weak_acid_ice
Ka=1e-5 → MISSING_REQUIRED_INPUT
Sources: OpenStax Chemistry 2e — Weak Acids — Equilibrium of a weak acid
Supports: Ka = [H+][A−]/[HA]; ICE from formal C; dilute water may include Kw
IUPAC Gold Book — acid dissociation constant — acidity constant
Supports: Ka is the equilibrium constant for HA ⇌ H+ + A−
Calculation version: 1.0.0

Background

Interpretation and common distinctions.

Compute monoprotic weak-acid ICE from formal concentration and Ka.

Supported and not supported

Supported — HA ⇌ H+ + A− from C and Ka or pKa; optional Kw at 25 °C · API chemistry.equilibrium.weak_acid

Not supported — Henderson–Hasselbalch, pH=−log10[H+] identities, polyprotic, weak base, activity coefficients, titration

Agent / API notes

Capability id: chemistry.equilibrium.weak_acid · tool id: weak-acid-equilibrium · pin 1.0.0.

{ "mode": "ice", "C": 0.1, "Ka": 1.8e-5 }

Alternate encoding: { "C": 0.1, "pKa": 4.76 }. Modes: ice (default, includes water), ice_no_water. Errors: MISSING_REQUIRED_INPUT, INVALID_NUMBER, VALUE_MUST_BE_POSITIVE, MIXED_INPUT_ENCODING, INVALID_MODE, NO_PHYSICAL_ROOT.

Calculator URL is /calc/chemistry/weak-acid-equilibrium. Buffer pH stays on /calc/chemistry/henderson-hasselbalch. Water-ion identities stay on /calc/chemistry/ph.

Other calculators in this family: Henderson–Hasselbalch, pH, Molarity .

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Frequently asked questions

Key distinctions behind the calculation.

Is this Henderson–Hasselbalch?

No. Buffer pH from a given [HA]/[A−] pair is /calc/chemistry/henderson-hasselbalch. This page solves ICE from formal C and Ka.

Is this pH = −log10[H+]?

No. That identity is /calc/chemistry/ph. This page finds [H+] from charge and mass balance, then reports pH.

Does this cover polyprotic acids or activity coefficients?

No. Monoprotic HA only. Weak base, polyprotic, activity, and titration are later capabilities.

Where does this run?

Locally in the browser by default. REST and MCP call the same chemistry-engine ICE wrapper, which reuses math.numerical.root.