Pka Calculator

Optional — used to calculate percent dissociation.
M
Chemistry

Calculate pKa from Ka (acid dissociation constant) or Ka from pKa, and estimate percent dissociation at a given pH.

Formula

pKa = −log₁₀(Ka); % dissociation = 100 / (1 + 10^(pKa−pH))
  • pKa = −log₁₀(Ka); Ka = 10^(−pKa).
  • Stronger acids have lower pKa (more negative).
  • % dissociation at a given pH uses Henderson-Hasselbalch principles.
  • Approximate % dissociation from concentration: α ≈ √(Ka/C) × 100 (valid when α < 5%).

Acetic acid: pKa = 4.75, C = 0.1 M, pH = 5

Inputs
  • Enter: pKa
  • Value: 4.75
  • Solution pH (for dissociation %): 5
  • Acid Concentration: 0.1 M

Ka = 10^−4.75 = 1.78×10⁻⁵. At pH 5: 64% dissociated.

Frequently asked questions

What does pKa tell you?
A lower pKa means a stronger acid (more easily donates a proton). pKa < 0: strong acid; pKa 2–5: moderately weak; pKa > 10: very weak acid.
What is the relationship between pH and pKa?
When pH = pKa, the acid is exactly half dissociated (50% each of HA and A⁻).