Ka Calculator. The Ka Calculator converts between an acid's dissociation constant and its logarithmic form — two values chemists use to measure how strongly an acid releases hydrogen ions in solution. Select your Calculation Mode: convert Ka to pKa, convert pKa to Ka, or find Ka from pH and initial acid concentration. Results include the Ka, pKa, acid strength classification, and the [H⁺] concentration. Also try the calculate Equivalence Point Calculated Value.
Ka (acid dissociation constant) measures how completely an acid dissociates in water. A larger Ka value indicates a stronger acid that donates protons more readily. Ka values typically range from 10⁻¹⁵ for very weak acids to greater than 1 for strong acids. See also our Buffer Capacity Calculator.
pKa is the negative logarithm of Ka: pKa = -log₁₀(Ka). This logarithmic scale makes it easier to work with very small Ka values. A lower pKa corresponds to a higher Ka and indicates a stronger acid.
For a monoprotic weak acid, use the equation: Ka = [H⁺]² / ([HA]₀ - [H⁺]), where [H⁺] = 10⁻ᵖᴴ and [HA]₀ is the initial acid concentration. This assumes the acid dissociation follows the equilibrium HA ⇌ H⁺ + A⁻.
pKa indicates acid strength on a logarithmic scale. Strong acids have pKa < 0, weak acids have pKa between 0-14, and very weak acids have pKa > 14. Each unit decrease in pKa represents a 10-fold increase in acid strength.
The Henderson-Hasselbalch equation (pH = pKa + log([A⁻]/[HA])) is derived from the Ka expression. It's useful for buffer calculations and shows how pH depends on the ratio of conjugate base to acid concentrations.
This calculator is designed for monoprotic acids (acids with one ionizable proton). Polyprotic acids like H₂SO₄ or H₃PO₄ have multiple Ka values (Ka1, Ka2, etc.) and require more complex calculations for each dissociation step.
Acetic acid: Ka = 1.8×10⁻⁵ (pKa = 4.74), formic acid: Ka = 1.8×10⁻⁴ (pKa = 3.74), hydrofluoric acid: Ka = 6.8×10⁻⁴ (pKa = 3.17). Strong acids like HCl have Ka >> 1. You might also find our Polyprotic Acid Calculator useful.