A weak acid HA does not fully ionize, and how far it gets is measured by its acid dissociation constant $K_a$ — a pure number, bigger for a stronger acid. Its conjugate base $\mathrm{A^-}$ is what is left once the proton has gone, and its own strength as a base is $K_b$, also a pure number. Those two are not independent. For a conjugate pair — and only for a pair — $K_a K_b = K_w$, read aloud K-a times K-b equals K-w, with $K_w = 1.0 \times 10^{-14}$ at 25 °C. A fixed product means a see-saw: the stronger the acid, the feebler the base it leaves behind.

Both constants sprawl across twenty orders of magnitude, so the p operator gets the same job it did on concentrations: $\mathrm{p}K_a = -\log K_a$ and $\mathrm{p}K_b = -\log K_b$. Take −log of the product relation and it becomes a sum, $\mathrm{p}K_a + \mathrm{p}K_b = 14$. Read the p-scale carefully, because it runs BACKWARDS: a lower pKa is a larger Ka is a stronger acid. Acetic acid's 4.76 beats hypochlorous acid's 7.54 by a factor of six hundred, and the smaller number is the winner.