Ka to pKa Converter
Convert between the acid dissociation constant Ka and pKa, both directions, with the conjugate base’s Kb and pKb.
Ka 1.8 × 10⁻⁵ converts to pKa 4.74.
To convert Ka to pKa, take the negative base-10 logarithm: pKa = −log₁₀(Ka). For acetic acid, Ka = 1.8 × 10⁻⁵ gives pKa = 4.74. Reverse it with Ka = 10⁻ᵖᴷᵃ. At 25 °C the conjugate base follows pKb = 14 − pKa, so pKb = 9.26 and Kb ≈ 5.6 × 10⁻¹⁰.
What Ka and pKa mean
The acid dissociation constant Ka measures how far a weak acid splits into ions in water: a larger Ka means more dissociation and a stronger acid. Because Ka values are tiny and span many orders of magnitude, chemists usually report pKa instead — its negative base-10 logarithm. Converting collapses awkward numbers like 1.8 × 10⁻⁵ into a tidy 4.74.
Reverse it with Ka = 10⁻ᵖᴷᵃ; at 25 °C, pKb = 14 − pKa
Worked example
Convert the Ka of acetic acid, 1.8 × 10⁻⁵, to pKa and find its conjugate base’s Kb.
- 1 Write Ka in scientific notation. Ka = 1.8 × 10⁻⁵ for acetic acid.
- 2 Take the negative base-10 logarithm. pKa = −log₁₀(1.8 × 10⁻⁵) = 4.74.
- 3 Find the conjugate base. At 25 °C, pKb = 14 − pKa = 14 − 4.74 = 9.26.
- 4 Convert pKb back to Kb. Kb = 10⁻⁹·²⁶ ≈ 5.6 × 10⁻¹⁰.
Ka and pKa of common weak acids
Aqueous values at 25 °C; a lower pKa marks a stronger acid.
| Acid | Ka | pKa |
|---|---|---|
| Hydrofluoric (HF) | 6.6 × 10⁻⁴ | 3.18 |
| Formic (HCOOH) | 1.8 × 10⁻⁴ | 3.75 |
| Acetic (CH₃COOH) | 1.8 × 10⁻⁵ | 4.74 |
| Carbonic (H₂CO₃, K₁) | 4.3 × 10⁻⁷ | 6.37 |
| Ammonium (NH₄⁺) | 5.6 × 10⁻¹⁰ | 9.25 |
Reading the numbers
Lower pKa = stronger acid. Because pKa is a negative logarithm, a smaller value means a larger Ka and more complete dissociation. HF (pKa 3.18) is a stronger weak acid than acetic acid (pKa 4.74), and each whole pKa unit is a tenfold change in Ka.
The Ka–Kb link. An acid and its conjugate base share a fixed relationship: Ka × Kb = Kw = 1.0 × 10⁻¹⁴ at 25 °C, which is the same as pKa + pKb = 14. A strong weak-acid therefore has a very weak conjugate base, and vice versa. Watch the temperature — the 14 comes from Kw, which drifts with temperature, so it holds exactly only at 25 °C.