Find a weak acid’s pKa from its acid dissociation constant (Ka).
How it works
pKa is defined as -log₁₀(Ka). Acetic acid, with a Ka of 1.8 × 10⁻⁵, has a pKa of about 4.74.
What this does not include
This is distinct from this site’s Henderson-Hasselbalch calculator, which takes pKa as a known input to find a buffer’s pH — this page instead derives pKa itself from the raw dissociation constant.
How to use this calculator
- Enter the acid’s Ka value.
A worked example
An acid dissociation constant (Ka) of 0.000018 → pKa = −log₁₀(Ka) = 4.7447.
Ka = 0.000063 → pKa = 4.2007 — a smaller pKa here, since a larger Ka means a stronger acid.
What the variables mean
| Variable | Meaning |
|---|---|
| Ka | The acid dissociation constant |
| pKa | The negative base-10 logarithm of Ka |
Edge cases worth knowing
A lower pKa means a stronger acid, not a weaker one — an easy point to get backwards, since pKa is an inverted logarithmic scale of acid strength.
A Ka of zero has no defined logarithm — the calculator declines to show a result rather than an undefined value.
Why use pKa instead of Ka directly?
Ka values span many orders of magnitude and are awkward to compare at a glance; the logarithmic pKa scale compresses that range into more manageable, comparable numbers, much like pH does for hydrogen ion concentration.
What does a lower pKa mean about acid strength?
A lower pKa means a larger Ka, which means the acid dissociates more completely — a stronger acid.
Can pKa be negative?
Yes — very strong acids (with Ka greater than 1) have a negative pKa, since the logarithm of a number greater than 1 is positive, and pKa is that logarithm’s negative.
