How this calculator works
For a weak acid and its conjugate base at equilibrium, rearrange Henderson-Hasselbalch to obtain pKa. Concentration units cancel when both species use the same base units. Alternatively take the negative base-10 logarithm of Ka. Thermodynamic constants use activities relative to standard states; the concentration version assumes a sufficiently dilute buffer and similar activity coefficients.
Formula
pKa = pH - log10([A-]/[HA]); pKa = -log10(Ka); Ka = 10^(-pKa).
Worked example
At pH 5 with equal acid and conjugate-base concentrations, pKa is 5 and Ka is 1e-5. At pH 6 with ten times as much conjugate base as acid, pKa is also 5.
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Frequently asked questions
Does a lower pKa mean a stronger acid?
For the same solvent and temperature, a lower pKa generally means a stronger acid.
Can either concentration be zero?
No. The logarithmic buffer relation requires both conjugate partners.
Is pKa always positive?
No. Strong acids can have negative pKa values.