⚗️ Weak Acid & Base pH Calculator
Enter the Ka (or Kb) and the concentration to get the pH of a weak acid or base — solved exactly, no "x is small" shortcut.
pH
2.88
pOH
11.12
[H⁺]
1.33e-3 M
% ionization
1.33%
Solves the equilibrium exactly via the ICE quadratic: for a weak acid, Ka = x² ÷ (C − x) with x = [H⁺], so x² + Ka·x − Ka·C = 0. No "x is small" approximation is made, so it stays accurate even for stronger weak acids or dilute solutions. pKa = −log Ka. Assumes a monoprotic acid/base at 25 °C (pKw = 14). 🔒 In your browser.
How the weak acid & base ph calculator works
A weak acid only partly ionises, so its pH isn't simply −log of the concentration. Setting up the ICE table for HA ⇌ H⁺ + A⁻ gives Ka = x² ÷ (C − x) where x = [H⁺]; the tool solves that quadratic exactly (x² + Ka·x − Ka·C = 0), then reports pH = −log x, along with pOH, the ion concentration and the percent ionization. Weak bases work the same way through Kb and pOH.
Because it solves the full quadratic rather than assuming x is negligible, it stays accurate for stronger weak acids and dilute solutions where the common approximation breaks down. It models a single (monoprotic) ionisation step at 25 °C (pKw = 14); for polyprotic acids it treats only the first dissociation. Enter Ka in decimal or scientific form (e.g. 1.8e-5).
Frequently asked questions
How do I find the pH of a weak acid?
From its Ka and concentration: set Ka = x²/(C − x) with x = [H⁺] and solve the quadratic, then pH = −log x. For 0.1 M acetic acid (Ka = 1.8×10⁻⁵) the pH is about 2.87 — not 1, as a strong acid would give.
Why can't I just take −log of the concentration?
That only works for strong acids, which ionise completely. A weak acid ionises partially, so the actual [H⁺] is far lower than the acid concentration — you must solve the equilibrium, which this tool does exactly.
What is percent ionization?
The fraction of the acid (or base) that actually dissociates: [H⁺] ÷ initial concentration × 100. Weak acids typically ionise only a few percent; it rises as the solution gets more dilute.
What is the difference between Ka and pKa?
pKa = −log Ka, a more convenient scale. A smaller pKa (larger Ka) means a stronger acid. Acetic acid's Ka of 1.8×10⁻⁵ is a pKa of about 4.74.
Does this work for weak bases?
Yes — switch to base mode and enter the Kb. It solves for [OH⁻], gives the pOH, then pH = 14 − pOH at 25 °C.