pKa to Ka Converter

The pKa to Ka Converter converts between pKa, Ka, pKb, and Kb for an acid-base conjugate pair. Enter any ONE of the four values and it derives the other three.

The formulas

Ka and Kb are dissociation constants; pKa and pKb are their negative base-10 logarithms, the same relationship pH has to hydrogen ion concentration:

Ka = 10^(-pKa)      pKa = -log10(Ka)
Kb = 10^(-pKb)      pKb = -log10(Kb)

For a conjugate acid-base pair — an acid and the base it forms after donating a proton (or vice versa) — in water at 25°C:

pKa + pKb = 14        (since Ka × Kb = Kw = 1.0 × 10⁻¹⁴)

That relationship is what lets the tool go from, say, an acid's pKa straight to its conjugate base's Kb: it computes the acid's own Ka first, subtracts pKa from 14 to get pKb, then converts that to Kb.

How to use it

1. Pick which value you're starting from — pKa, Ka, pKb, or Kb.
2. Enter it. Ka and Kb accept scientific notation (1.8e-5 works the same as 0.000018).
3. The result table shows all four values, with your entry marked Input and the rest Derived.

Example

Acetic acid has a pKa of 4.76:

Ka = 10^(-4.76) = 1.74 × 10⁻⁵
pKb = 14 - 4.76 = 9.24
Kb = 10^(-9.24) = 5.75 × 10⁻¹⁰

The 5.75 × 10⁻¹⁰ Kb belongs to acetate (CH₃COO⁻), acetic acid's conjugate base — not to acetic acid itself. Ka and Kb always describe two different species in the pair, never the same molecule.

Why does pKa + pKb equal 14?

At 25°C, water's autoionization constant Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴. For a conjugate pair, multiplying the acid's Ka by the base's Kb always gives Kw, regardless of the pair — so taking -log10 of both sides gives pKa + pKb = 14. This only holds at 25°C; at another temperature, Kw (and so the 14) changes.

Can pKa be negative?

Yes — strong acids like HCl have Ka values well above 1, which makes pKa negative (HCl's is around -7). The tool accepts any real number for pKa or pKb; only Ka and Kb are required to be positive, since they're concentration ratios.

To find the molar mass needed to prepare a solution of a known concentration, use the Molar Mass Calculator. To go from moles and solvent mass to molality instead, use the Molality Calculator.

Privacy

All calculations run entirely in your browser. Your values are never uploaded or stored.