Why does this use the quadratic formula instead of the approximation?

The approximation (x ≈ √(Ka·C)) is only valid when dissociation is below ~5%. This calculator always uses the exact quadratic solution so it works for strong weak acids and dilute solutions where the approximation breaks down.

What is the degree of dissociation?

It is the fraction (or percent) of acid molecules that have donated a proton. A value of 5% means 5 out of every 100 HA molecules dissociated into H+ and A−.

What Ka values are typical for weak acids?

Weak acids have Ka roughly between 10⁻² (strong weak acid, e.g. HSO₄⁻, Ka ≈ 0.012) and 10⁻¹⁴ (very weak). Acetic acid is 1.8×10⁻⁵, carbonic acid 4.3×10⁻⁷.

Can I use this for polyprotic acids?

Yes, for the first dissociation step. Enter Ka1. The subsequent dissociation steps contribute negligibly to [H+] for most polyprotic acids and can be treated separately.

Why is pH higher when I dilute the acid?

Dilution lowers [H+] even though percent dissociation increases. The net effect on pH depends on the interplay between C and Ka; this calculator handles both correctly.

Acid Dissociation Calculator

Name: Acid Dissociation Calculator
Availability: InStock
Author: Nham Vu

Enter a weak acid's Ka and initial concentration to find pH, [H+], and percent dissociation.

Weak Acid Parameters

Acid Dissociation Constant (Ka)

× 10 (exponent)

Example: acetic acid Ka = 1.8 × 10⁻⁵

Initial Concentration C (mol/L)

mol/L

Quick Presets

Results

Enter Ka and concentration, then click Calculate.

How the Math Works

Equilibrium

HA ⇌ H⁺ + A⁻

Ka = x² / (C − x)

Quadratic Form

x² + Ka·x − Ka·C = 0

x = [H⁺] (take + root)

Derived Values

pH = −log₁₀(x)

α = (x / C) × 100%

Summary

Enter a weak acid's Ka and initial concentration to find pH, [H+], and percent dissociation.

How it works

Enter the acid dissociation constant (Ka) for your weak acid.
Enter the initial concentration (C) of the acid in mol/L.
The calculator solves the quadratic equation x² + Ka·x − Ka·C = 0 for x = [H+].
pH is computed as −log₁₀([H+]).
Percent dissociation is (x / C) × 100%.

Use cases

Determine pH of a weak acid solution in chemistry coursework.
Verify the 5% approximation rule for simplifying Ka calculations.
Compare dissociation across different acid concentrations.
Calculate [H+] for buffer preparation at a target pH.
Analyze acid strength from Ka values in lab reports.
Quick sanity-check during titration experiment planning.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu