How to use this calculator
- Select pH, pKa, acid amount, or base amount as the unknown.
- Enter the other three values.
- Calculate and check the base-to-acid ratio.
Formula and variables
pH = pKa + log10([A-]/[HA])
| Symbol | Meaning | Unit |
|---|---|---|
| pKa | acid dissociation exponent | dimensionless |
| [A-] | conjugate base | concentration or amount |
| [HA] | weak acid | concentration or amount |
Worked example
Equal acid and conjugate-base amounts give pH = pKa because log10(1) = 0.
Interpretation
The equation applies to a conjugate acid-base buffer pair under the disclosed activity approximation.
Common mistakes
- Acid and base amounts must be positive.
- Do not apply this equation far outside the buffer region without assessing assumptions.
Frequently asked questions
Can I use moles instead of concentrations?
Yes. Use moles when both buffer components share the same final volume.
When does pH equal pKa?
They are equal when the conjugate base and weak acid amounts are equal.
When is the equation unreliable?
It is unreliable after buffer exhaustion and when activity effects are significant.
Can I use scientific notation?
Yes. Enter values such as 6.022e23 or 1.0e-6 in a numeric field.
How should I round the result?
Keep extra digits during the calculation. Round the final result to the precision required by your data.