Determine the acid dissociation constant, Ka
Use the formula pKa = -log10(Ka)
If given pH and concentrations, use the Henderson-Hasselbalch equation: pH = pKa + log10([A-]/[HA])
Rearrange the Henderson-Hasselbalch equation to solve for pKa: pKa = pH – log10([A-]/[HA])
If given titration data, find the half-equivalence point
At the half-equivalence point, pH = pKa
If given Ka from an equilibrium expression, calculate pKa directly with pKa = -log10(Ka)
If given pKb for a base, use pKa + pKb = 14 at 25°C
Use experimental data from a titration curve, buffer solution, or equilibrium calculation to identify the needed values