Acid Dissociation Constant Calculator

Work out acid dissociation constant instantly with clear inputs, formula shown and shareable results.

About Acid Dissociation Constant Calculator

For a weak monoprotic acid, the pH gives [H+] directly, and that same figure is the concentration of conjugate base formed. Ka is then [H+]^2 divided by the acid remaining, C - [H+]. A 0.1 M acid reading pH 2.87 has [H+] = 1.35e-3 and Ka near 1.8e-5, the value for acetic acid.

Formulas

Equilibrium expression

Ka = [H+][A-] / [HA] = [H+]^2 / (C - [H+])

From pH

[H+] = 10^-pH

Worked example

With the default values below, Acid Dissociation Constant Calculator returns Acid Dissociation Constant Ka of 1.8446 × 10⁻⁵. Change any input above and every figure here is recalculated in your browser.

Inputs used

Initial acid concentrationminimum 0 mol/L: 0.1 mol/L
Measured pHaccepts -2–14: 2.87

Results

Acid Dissociation Constant Ka: 1.8446 × 10⁻⁵
pKa: 4.734
Degree Of Ionisation: 1.349%
Hydrogen Ion Concentration: 1.3490 × 10⁻³

Step by step

1Values used
2Equilibrium expression
3From pH
4Acid Dissociation Constant Ka= 0.0000
5pKa= 4.734
6Degree Of Ionisation= 1.349
7Hydrogen Ion Concentration= 0.0013

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why subtract [H+] from the concentration?

Because every hydrogen ion released came from an acid molecule, so the undissociated acid left is the initial concentration minus that amount.

When can I skip the subtraction?

When ionisation is under about 5%, the approximation Ka = [H+]^2/C is within experimental error. This calculator does the exact form regardless.