pKa Calculator
Use this pKa calculator to solve the Henderson-Hasselbalch relation or convert directly between Ka and pKa values.
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Run the calculator.
What This pKa Calculator Helps You Do
This calculator covers the two main ways pKa shows up in practice: direct conversion from Ka and buffer-style back-calculation from pH plus the acid-base ratio.
That makes it useful for acid-strength comparisons, Henderson-Hasselbalch exercises, and quick buffer checks where pKa is the value you actually need to reason with.
How to Calculate pKa Calculator
- Choose the conversion path: Use Henderson-Hasselbalch mode when pH and acid/base concentrations are known, or use the direct Ka and pKa conversions.
- Enter consistent values: Concentrations must use the same units when they appear in the ratio [A-]/[HA].
- Interpret acidity strength: Lower pKa generally indicates a stronger acid, while higher pKa indicates a weaker acid.
pKa Calculator Formula
| Variable | Meaning | Unit |
|---|---|---|
| pKa | Negative base-10 log of the acid dissociation constant | unitless |
| Ka | Acid dissociation constant | unitless |
| [A-] | Concentration of conjugate base | mol/L |
| [HA] | Concentration of weak acid | mol/L |
| pH | Solution pH | unitless |
Use the worked examples below to check how the formula behaves with real values. If the result looks unexpected, verify the unit assumptions and the meaning of each variable before interpreting the answer.
Worked Examples
- pH: 4.50
- Conjugate base: 0.58 M
- Weak acid: 1.00 M
Result: pKa = 4.7366
The Henderson-Hasselbalch equation gives a pKa close to acetic acid.
- Ka: 1.8e-5
Result: pKa = 4.7447
The negative logarithm of Ka gives the pKa directly.
How to Interpret Your Results
| Range | Meaning | Action |
|---|---|---|
| pKa < 0 | Very strong acid behavior | These acids dissociate extensively in water. |
| 0 to 7 | Common weak-acid range | Typical for many buffer and organic-acid problems. |
| > 7 | Very weak acid | Large pKa values indicate limited dissociation. |
Frequently Asked Questions
References
Last reviewed: March 2026