pKa is one of the most-used numbers in acid-base chemistry — it tells you at a glance how strong an acid is and, combined with a buffer's composition, how to predict the pH of a solution. This calculator converts between Ka and pKa, between Kb and pKb, and computes buffer pH from the Henderson-Hasselbalch equation.
Why use pKa instead of Ka
Acid dissociation constants (Ka) for weak acids span an enormous range — from around 10⁻² for moderately strong acids down to 10⁻¹⁰ or smaller for very weak ones. Comparing numbers like 1.8×10⁻⁵ and 4.9×10⁻₁⁰ by eye is awkward. Taking the negative log10 compresses that range into small, easy-to-compare numbers: pKa = −log₁₀(Ka). A lower pKa always means a stronger acid, and each whole-number drop in pKa represents a tenfold increase in Ka.
Ka, Kb, and the pKa + pKb = 14 relationship
Every acid has a conjugate base, and every base has a conjugate acid. In water at 25°C, the acid dissociation constant of the acid and the base dissociation constant of its conjugate base are linked through the ion-product constant of water, Kw = 1.0×10⁻¹⁴, which gives pKa + pKb = 14. This means if you know pKa for an acid, you automatically know pKb for its conjugate base — and vice versa — without any additional data.
Predicting buffer pH with Henderson-Hasselbalch
A buffer made from a weak acid and its conjugate base resists pH changes because added H⁺ or OH⁻ gets absorbed by the acid/base pair rather than changing the free H⁺ concentration much. The Henderson-Hasselbalch equation, pH = pKa + log₁₀([base]/[acid]), lets you calculate the resulting pH directly from the acid's pKa and the ratio of conjugate base to acid concentrations — no equilibrium-constant algebra required. When the ratio is 1 (equal concentrations), the log term is zero and pH simply equals pKa, which is why chemists often pick a buffer whose pKa is close to their target pH.