Ka is the acid dissociation constant, a measure of how completely an acid donates its proton (H⁺) in aqueous solution. A larger Ka means a stronger acid. For HA ⇌ H⁺ + A⁻, Ka = [H⁺][A⁻] / [HA].

What units does Ka have?

Ka is technically dimensionless when activities are used, but in practice it is expressed as mol/L (M) because the standard reference state for solutes is 1 M.

How is pH calculated from Ka?

Using the quadratic approximation: [H⁺] = (-Ka + √(Ka² + 4·Ka·C)) / 2, then pH = -log₁₀([H⁺]). For very weak acids (Ka ≪ C) the simpler formula [H⁺] ≈ √(Ka·C) is also shown.

Why does the tool use the quadratic formula?

The simple approximation [H⁺] ≈ √(Ka·C) fails when Ka is large relative to C (i.e., dissociation > ~5%). The quadratic gives an accurate result for any Ka and concentration.

Can I use this for polyprotic acids?

This tool handles the first dissociation step only. For polyprotic acids (H₂SO₄, H₃PO₄, etc.) you would need to apply the Ka₁, Ka₂, … steps separately.

What is a typical Ka range for weak acids?

Weak acids typically have Ka values between 10⁻² and 10⁻¹², corresponding to pKa values of 2 to 12. Acetic acid has Ka ≈ 1.8 × 10⁻⁵ (pKa 4.74).

Ka Calculator

Name: Ka Calculator
Availability: InStock
Author: Nham Vu

Calculate Ka from equilibrium concentrations, or find the pH of a weak acid solution from its Ka and initial concentration.

For the reaction HA ⇌ H⁺ + A⁻, enter equilibrium concentrations:

[H⁺] — hydronium concentration (mol/L)

[A⁻] — conjugate base concentration (mol/L)

[HA] — undissociated acid concentration (mol/L)

Enter values and click Calculate to see results.

Summary

Calculate Ka from equilibrium concentrations, or find the pH of a weak acid solution from its Ka and initial concentration.

How it works

Choose a calculation mode: "Find Ka" or "Find pH from Ka".
In Find Ka mode, enter the equilibrium concentrations of [H⁺], [A⁻], and [HA].
In Find pH mode, enter the Ka value and the initial acid concentration (C).
Click Calculate to see the result along with the full equilibrium expression.
Use Copy to clipboard to transfer the result to another application.

Use cases

Determine Ka of acetic acid, formic acid, or any weak acid from lab measurements.
Predict the pH of a buffer or weak acid solution given its Ka.
Verify equilibrium constant calculations for chemistry homework.
Compare acid strengths by computing Ka for different acids.
Support analytical chemistry titration analysis.
Study acid-base equilibria in biochemistry and pharmacology.

Frequently Asked Questions

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Last updated: 2026-05-28 · Reviewed by Nham Vu