Ka Calculator

Calculate acid dissociation constant Ka from pH and initial concentration. Find degree of dissociation, percent ionization, and determine if acid is weak or strong.

🔬 Acid-Base📐 Ka = [H⁺][A⁻]/[HA]🧪 Chemistry
Initial acid concentration C (mol/L)
pH of solution
Please enter valid values.

Formula & Reference

VariableSymbolFormulaUnits
Ka Calculator—Ka = [H⁺][A⁻]/[HA]mol/L

Step-by-Step Examples

Example 1
Acetic Acid

0.100 M CH3COOH has pH = 2.87.

  • [H+] = 10^-2.87 = 1.35x10^-3 M
  • Ka = (1.35x10^-3)^2/(0.100-1.35x10^-3)
  • Ka = 1.82x10^-6/0.09865 = 1.85x10^-5
✓ Ka = 1.85x10^-5 (lit: 1.8x10^-5)
Example 2
Unknown Weak Acid

0.200 M unknown acid has pH = 3.20.

  • [H+] = 10^-3.20 = 6.31x10^-4
  • Ka = (6.31x10^-4)^2/(0.200-6.31x10^-4) = 3.98x10^-7/0.1994
  • Ka = 2.0x10^-6
✓ Ka = 2.0x10^-6
Example 3
Chloroacetic Acid

0.100 M ClCH2COOH, pH = 1.95.

  • [H+] = 10^-1.95 = 0.01122
  • Ka = (0.01122)^2/(0.100-0.01122) = 1.259x10^-4/0.08878
  • Ka = 1.42x10^-3
✓ Ka = 1.42x10^-3 (pKa = 2.85)

Real-World Applications

🔬
Acid-Base Chemistry
Ka fundamentally characterizes acid strength and determines pH of solutions.
🧪
Analytical Chemistry
Ka values used to predict buffer behavior and titration curves.
💊
Drug Design
pKa of drug molecule affects absorption across biological membranes.
🌊
Ocean Chemistry
CO2 dissolution: Ka for H2CO3 determines ocean pH as CO2 increases.

Common Mistakes to Avoid

⚠️
The approximation [HA] ~ C - [H+]

Use x = [H+] = [A-]. Ka = x^2/(C-x). If Ka << C: simplify to x^2/C. Check: if x > 5% of C, do not simplify.

⚠️
pH must give [H+] < C

If calculated [H+] > initial concentration C, you have a strong acid or the pH is wrong.

⚠️
Use exact formula for accuracy

Ka = [H+]^2/(C-[H+]). Do not assume [H+] << C unless explicitly verified to be less than 5%.

Frequently Asked Questions

What is Ka? ▾
Equilibrium constant for acid dissociation: HA to H+ + A-. Ka = [H+][A-]/[HA]. Large Ka: strong acid. Small Ka: weak acid.
How do I find Ka from titration data? ▾
At half-equivalence point in titration: pH = pKa (Henderson-Hasselbalch with equal moles acid and conjugate base).
What is the percent ionization? ▾
% ionization = ([H+]/C) x 100%. For weak acids: increases as C decreases (dilution increases percent ionization but decreases absolute [H+]).
What are Ka values for common acids? ▾
HCl: Ka ~ 10^7 (strong). HF: 6.8x10^-4. Acetic: 1.8x10^-5. H2CO3: 4.3x10^-7. HCN: 6.2x10^-10. Boric acid: 5.8x10^-10.
How does Ka relate to Kb for conjugate base? ▾
Ka x Kb = Kw = 10^-14 at 25 C. If Ka(HA) = 1.8x10^-5, then Kb(A-) = 10^-14/1.8x10^-5 = 5.6x10^-10.

Related Chemistry Calculators

🔬 pKa Calculator🔬 Kb Calculator🧪 Henderson-Hasselbalch🔬 pH Calculator🔬 Weak Acid pH

Formula Explorer connections

Interpretation: This relationship connects hydrogen-ion activity, dissociation, conjugate ratios or titration stoichiometry to acid–base behavior. Assumption: Concentration approximates activity mainly in dilute solutions. Temperature, ionic strength, acid strength and equilibrium approximations affect accuracy.

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