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A Level

Weak Acid and Buffer Calculator

A free teacher tool for exploring weak-acid equilibria, buffer calculations and half-equivalence points with the chemical reasoning and mathematical working kept visible.

What the calculator does

  • Calculates the pH of a weak acid from its concentration and Ka.
  • Provides the exact quadratic treatment when the usual approximation is unsuitable.
  • Calculates the pH of an acid–conjugate-base buffer.
  • Demonstrates why pH = pKa at half-equivalence.
  • Links masses and moles to buffer composition.
  • Displays every stage of the working.

Key relationships

Ka = [H+][A−] ÷ [HA]
pH = pKa + log([A−] ÷ [HA])

At half-equivalence, equal amounts of the weak acid and its conjugate base are present, so pH = pKa.

Open the calculator

Choose the weak-acid, buffer or half-equivalence calculation.

Launch Weak Acid and Buffer Calculator

The calculator opens in a new browser tab.

For structured A-level chemistry teaching and guided mastery, visit ACE Chemistry Academy.

How to Use the Weak Acid and Buffer Calculator

Choose the calculation required:

  • pH of a weak acid solution
  • pH of a buffer solution
  • half-equivalence point

Calculating the pH of a weak acid

1. Enter the acid data

Enter:

  • the initial concentration of the weak acid in mol dm⁻³;
  • either the value of Kₐ or pKₐ, as requested.

If pKₐ is supplied, the calculator uses:

Kₐ = 10⁻ᵖᴷᵃ

2. Check the equilibrium

For a weak acid, HA:

HA(aq) ⇌ H⁺(aq) + A⁻(aq)

The acid is only partially ionised. The hydrogen-ion concentration is therefore not normally equal to the initial acid concentration.

3. Follow the calculation

The calculator applies the acid-dissociation expression:

Kₐ = [H⁺][A⁻] ÷ [HA]

For a weak acid of concentration (c), the usual approximation gives:

[H⁺] ≈ √(Kₐc)

It then calculates:

pH = −log₁₀[H⁺]

Where appropriate, compare the amount ionised with the initial concentration to check whether the weak-acid approximation is reasonable.

Calculating the pH of a buffer

Enter:

  • Kₐ or pKₐ for the weak acid;
  • the concentration or amount of the weak acid, HA;
  • the concentration or amount of its conjugate base, A⁻.

The calculator uses:

pH = pKₐ + log₁₀([A⁻] ÷ [HA])

If both substances are present in the same total volume, their amounts in moles may be used in the ratio instead of their concentrations.

Half-equivalence point

At the half-equivalence point, half of the original weak acid has been converted into its conjugate base.

Therefore:

[HA] = [A⁻]

and:

pH = pKₐ

Check the reasoning

Before accepting the answer, check that:

  • the acid is weak rather than strong;
  • concentrations are in mol dm⁻³;
  • Kₐ and pKₐ have not been confused;
  • the correct acid–base pair has been used;
  • logarithms and units have been handled correctly.

A weak acid must not be treated as completely ionised. The calculator is intended to show the equilibrium reasoning, not merely provide a pH value.

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