ClarityKitFind a tool
Chemistry

Weak Acid pH Calculator

Estimate pH and percent ionization for a single monoprotic weak acid using the quadratic equilibrium expression.

Explicit units and model assumptionsLocal calculations with worked examples
Chemistry workspace
Chemistry workspace

Check the units, concentration basis and assumptions before interpreting the result.

See the method

Set up your calculation

Runs on your device

Your chemistry result

Your result

Let’s calculate.

Use the Calculate button to see your result.

Review the method below for assumptions and conventions.

How to use this tool

  1. Enter acid dissociation constant ka, initial acid concentration.
  2. Select Calculate to view the result.
  3. Check the method and assumptions below before using the result.

The method, explained

Solve x² + Ka·x − Ka·C = 0 with x = 2KaC/(sqrt(Ka² + 4KaC) + Ka), then pH = −log10(x). The stable form avoids subtractive cancellation.

A WORKED EXAMPLE

Using acid dissociation constant ka = 0.000018, initial acid concentration = 0.1 mol/L, the result is 2.87527706 pH. Change these example inputs to match your task; use the method above to check each step.

Understanding your result

No. It solves the quadratic rather than assuming dissociation is negligible relative to the initial concentration. Water and activity effects remain outside the model.

What to keep in mind

Neglects water autoionization and activity effects. Rejects predicted [H+] below 10⁻⁶ mol/L. Not for mixtures or polyprotic acids. Educational calculation only; use validated procedures for laboratory work.

Reference: OpenStax Chemistry 2e — chemical quantities and models

Common questions

Does this use the square-root approximation?

No. It solves the quadratic rather than assuming dissociation is negligible relative to the initial concentration. Water and activity effects remain outside the model.

Are my inputs uploaded?

No. This calculation runs in your browser without an account or an external API.

Methodology maintained by ClarityKit. How these tools are built and checked.