Ka and pH Calculator (Acid Dissociation)
Calculate Ka from pH and concentration, pH from Ka, or buffer pH using Henderson-Hasselbalch.
Includes ICE table, percent ionization, and common weak acid Ka table.
Acid Dissociation Constant (Ka) and pH
A weak acid partially dissociates in water. The equilibrium between the undissociated acid (HA) and its dissociation products (H⁺ and A⁻) is described by the acid dissociation constant Ka.
Dissociation Equilibrium HA ⇌ H⁺ + A⁻
Ka = [H⁺][A⁻] / [HA]
A larger Ka means a stronger acid (more dissociation). pKa = −log₁₀(Ka) provides a more convenient scale — a lower pKa means a stronger acid.
pH from Ka and Concentration (C) Using an ICE table (Initial, Change, Equilibrium):
- Initial: [HA] = C, [H⁺] = 0, [A⁻] = 0
- Change: [HA] = C − x, [H⁺] = x, [A⁻] = x
- Equilibrium: Ka = x² / (C − x)
Simplified (when x « C): x ≈ √(Ka × C), valid when x < 5% of C. Exact: solve quadratic x² + Ka·x − Ka·C = 0 → x = (−Ka + √(Ka² + 4·Ka·C)) / 2
Then pH = −log₁₀(x)
Ka from pH and Concentration If pH is known: [H⁺] = x = 10^(−pH), then Ka = x² / (C − x)
Buffer pH — Henderson-Hasselbalch Equation For a buffer containing a weak acid and its conjugate base: pH = pKa + log₁₀([A⁻] / [HA])
This is the Henderson-Hasselbalch equation, fundamental to biological and analytical chemistry.
Common Weak Acids Acetic acid (vinegar): Ka = 1.8 × 10⁻⁵, pKa = 4.74 Carbonic acid: Ka = 4.3 × 10⁻⁷, pKa = 6.37 Phosphoric acid (1st): Ka = 7.5 × 10⁻³, pKa = 2.12 Hydrofluoric acid: Ka = 6.8 × 10⁻⁴, pKa = 3.17