Ka Calculator

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.

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.