What is the Henderson-Hasselbalch equation?

pH = pKa + log([A⁻]/[HA]), where pKa is the acid dissociation constant of the weak acid, [A⁻] is the molar concentration of the conjugate base, and [HA] is the molar concentration of the undissociated weak acid.

Can I enter moles instead of molar concentrations?

Yes. Because the volume is the same for both species in the same solution, the [A⁻]/[HA] ratio equals the mole ratio n(A⁻)/n(HA). This tool accepts either unit.

Why does this tool show every calculation step?

Many students and researchers need to verify their manual work or understand where a number came from. The step-by-step panel shows the ratio, its log, and the final pH so every intermediate value is visible.

What is the valid pH range for this equation?

The Henderson-Hasselbalch equation is most accurate when both [A⁻] and [HA] are much larger than Kw/[H⁺] — roughly pH 4–10 for most buffers. At extreme pH values, the approximation breaks down and a full equilibrium calculation is needed.

What happens if I enter equal amounts of acid and base?

When [A⁻]/[HA] = 1, log(1) = 0, so pH = pKa. This is the half-equivalence point and the center of the effective buffering range.

Where do I find the pKa of my acid?

Common values: acetic acid 4.74, formic acid 3.75, carbonic acid (pKa1) 6.35, phosphoric acid (pKa2) 7.20, ammonium 9.25, Tris 8.07, HEPES 7.55. Look up NIST or your reagent datasheet for others.

pH of Buffer Solution (Henderson-Hasselbalch)

Enter pKa and the moles (or concentrations) of conjugate base and weak acid to calculate buffer pH with a full step-by-step solution.

Henderson-Hasselbalch Inputs

pH = pKa + log([A⁻] / [HA])

pKa of weak acid

Acetic acid 4.74 · Phosphate (pKa2) 7.20 · Tris 8.07 · NH₄⁺ 9.25

Input unit

Moles (mol) Concentration (mol/L)

Conjugate base [A⁻] (mol)

Weak acid [HA] (mol)

Summary

This tool applies the Henderson-Hasselbalch equation — pH = pKa + log([A⁻]/[HA]) — to find the exact pH of a buffer solution and shows every intermediate step. Enter the pKa of your weak acid and either molar concentrations or mole amounts of the conjugate base and weak acid. The step-by-step panel shows the ratio, its log, the addition to pKa, and the final pH, making it useful for checking homework or understanding buffer behavior.

How it works

Enter the pKa of the weak acid (e.g., 4.74 for acetic acid, 6.35 for carbonic acid H₂CO₃).
Choose whether you are entering molar concentrations (mol/L) or mole amounts (mol) — the ratio is the same either way.
Enter the amount of conjugate base [A⁻] and weak acid [HA].
Click Calculate to see the buffer pH plus a line-by-line solution panel.
The solution shows: the [A⁻]/[HA] ratio, log₁₀ of that ratio, and the final pH = pKa + log([A⁻]/[HA]).
Adjust any input and the result updates instantly — no page reload needed.

Use cases

Verify homework problems on buffer pH calculations.
Prepare lab buffers by confirming the expected pH before mixing.
Understand how changing the base-to-acid ratio shifts pH up or down.
Teach or learn the Henderson-Hasselbalch equation step by step.
Check buffer calculations for acetate, phosphate, carbonate, or custom acid systems.
Convert mole amounts to a pH without manually computing logarithms.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu