Percent Ionization Calculator
Calculate percent ionization of weak acids and bases from Ka or Kb and initial concentration.
Shows full ICE table, Ostwald's dilution law, and approximation validity.
Percent Ionization of Weak Acids and Bases
A weak acid or base only partially dissociates in water. The fraction that does ionize is expressed as percent ionization (also called degree of dissociation or α).
Weak Acid Ionization For a weak acid HA dissociating: HA ⇌ H⁺ + A⁻
Using an ICE table with initial concentration C and degree of ionization x: Ka = x² / (C − x)
Simplified (approximation): x ≈ √(Ka × C) — valid when x < 5% of C.
Exact (quadratic formula): x = (−Ka + √(Ka² + 4·Ka·C)) / 2
Percent ionization = (x / C) × 100%
Weak Base Ionization For a weak base B: B + H₂O ⇌ BH⁺ + OH⁻
Kb = x² / (C − x) — same form as Ka. Solves for x = [OH⁻], then: pOH = −log(x), pH = 14 − pOH.
Ostwald’s Dilution Law As concentration C decreases (dilution), percent ionization increases. This is Ostwald’s Dilution Law: α = √(Ka / C) for the simplified case.
Practical consequence: diluting a weak acid solution increases percent ionization — a more dilute solution is relatively more ionized, though the absolute concentration of H⁺ decreases.
The 5% Rule The simplified formula (ignoring x in the denominator) is valid when percent ionization < 5%. When ionization exceeds 5%, use the quadratic formula for an accurate result. This typically occurs when Ka is large relative to C (very weak concentrations or moderately strong weak acids).