pKa is the negative base-10 logarithm of the acid dissociation constant: pKa = −log10(Ka). Acetic acid has Ka = 1.8 × 10⁻⁵, so its pKa is 4.74. From a buffer, rearrange Henderson–Hasselbalch: pKa = pH − log10([A⁻]/[HA]); a pH 5.00 buffer with twice as much base as acid has pKa 4.70.
Students working acid–base problems and lab staff choosing a buffer both need pKa rather than the raw Ka, because pKa lines up directly with pH. This calculator takes either a Ka written in scientific notation or a measured buffer pH with the concentrations of the conjugate base and the weak acid. It returns the pKa to two decimals, the matching Ka, the pKb of the conjugate base at 25 °C and a rough strength label, and it shows each logarithm step. The buffer route relies on the Henderson–Hasselbalch approximation, which uses concentrations instead of activities, so it drifts at high ionic strength and when the base-to-acid ratio falls outside about 0.1 to 10.
Choose Ka value to convert a tabulated constant, or Buffer pH and ratio to work from a measured buffer.
Type the coefficient (1.8) and the power of ten (−5) for Ka = 1.8 × 10⁻⁵.
Give the measured pH and the molar concentrations of the conjugate base and the weak acid.
The pKa, Ka, pKb and strength label update as you type, with the logarithm steps listed below.
Result pKa: 4.74
Ka: 1.80 × 10^-5
pKb of the conjugate base (25 °C): 9.26
Acid strength: Weak acid
The tabulated Ka of acetic acid converts to the familiar pKa of 4.74, so an acetate buffer works best near pH 4.7.
Result pKa: 9.31
Ka: 4.90 × 10^-10
pKb of the conjugate base (25 °C): 4.69
Acid strength: Weak acid
A much smaller Ka gives a pKa above 9: HCN barely dissociates in water.
Result pKa: 4.70
Ka: 2.00 × 10^-5
pKb of the conjugate base (25 °C): 9.30
Acid strength: Weak acid
With twice as much conjugate base as acid, the pKa sits 0.30 units below the measured pH.
pKa = −log10(Ka); pKa = pH − log10([A⁻] / [HA]); pKb = 14 − pKa
KapH[A⁻][HA]Ka is entered as coefficient × 10^n, so the logarithm is log10(coefficient) + n, which avoids typing long strings of zeros. In buffer mode the Henderson–Hasselbalch equation is solved for pKa. The pKb uses Kw = 1.0 × 10⁻¹⁴, valid at 25 °C only; at 37 °C pKw is closer to 13.6.
Because pKa is a negative logarithm. A larger Ka means more dissociation, and −log10 of a larger number is smaller. Each one-unit drop in pKa is a tenfold increase in Ka: formic acid (pKa 3.75) is about ten times stronger than acetic acid (pKa 4.74).
Raise ten to the negative pKa: Ka = 10^(−pKa). A pKa of 9.31 gives 10^−9.31 ≈ 4.9 × 10⁻¹⁰, the Ka of hydrocyanic acid. The Ka line in the results does this conversion for whatever pKa you get.
When the conjugate base and weak acid concentrations are equal, log10(1) = 0 and the pH equals the pKa. That is the half-equivalence point of a titration, and the point where the buffer resists pH change best.
For a conjugate acid–base pair in water, pKa + pKb = pKw, which is 14.00 at 25 °C. The acetate ion therefore has pKb = 14 − 4.74 = 9.26. The relation shifts with temperature because pKw does.
Henderson–Hasselbalch assumes the acid and base concentrations barely change on dissociation. Once one form outnumbers the other more than tenfold, the minor species is affected noticeably by water's own equilibrium and by dissociation, so the derived pKa becomes less trustworthy.
Yes. Strong acids such as hydrochloric acid have Ka well above 1, so their pKa is below zero (around −6 for HCl). In water they are fully dissociated, and their pKa values come from indirect measurements in other solvents.
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Tools Directory Editorial Team. (2026, October 11). pKa Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/chemistry/chemistry-reference/pka-calculator/
Tools Directory Editorial Team. “pKa Calculator.” Tools Directory, 11 Oct. 2026, https://tools.milunir.com/chemistry/chemistry-reference/pka-calculator/. Accessed 27 Oct. 2026.
Tools Directory Editorial Team. “pKa Calculator.” Tools Directory. Last modified October 11, 2026. Accessed October 27, 2026. https://tools.milunir.com/chemistry/chemistry-reference/pka-calculator/.
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