Ka is the acid dissociation constant. For HA ⇌ H⁺ + A⁻ at equilibrium, Ka = [H⁺][A⁻]/[HA]. Smaller Ka means a weaker acid.

When can I use the approximation [H⁺] ≈ √(Ka × C)?

When ionisation is less than 5% (typically when Ka < C/100). This calculator solves the full quadratic so it works in all cases.

pKa = -log₁₀ Ka. Acetic acid has Ka = 1.8 × 10⁻⁵, so pKa ≈ 4.74. The smaller the pKa, the stronger the acid.

How is this different from a strong acid?

A strong acid (HCl, HNO₃) fully ionises, so [H⁺] = initial concentration. A weak acid only partly ionises, so [H⁺] is much less and pH depends on Ka.

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Weak acid pH calculator

Q: What is pKa?

pKa = -log₁₀ Ka. Acetic acid has Ka = 1.8 × 10⁻⁵, so pKa ≈ 4.74. The smaller the pKa, the stronger the acid.

Q: How is this different from a strong acid?

A strong acid (HCl, HNO₃) fully ionises, so [H⁺] = initial concentration. A weak acid only partly ionises, so [H⁺] is much less and pH depends on Ka.

Enter Ka and initial concentration. Get the equilibrium [H⁺], pH, pKa, and percentage ionisation.

Inputs

Acid presetKaAcid dissociation constantInitial concentration [HA] (mol/L)

Result

2.875

[H⁺]

0.001333mol/L

pKa

4.745

% ionisation

1.333%

Solved exactly from Ka = x²/(C - x). For Ka ≪ C the approximation [H⁺] ≈ √(Ka × C) gives the same pH to 2 decimal places.

How this calculator works

It solves the quadratic Ka = x²/(C − x) exactly, so the answer is correct even when the small-x approximation fails. The result includes percentage ionisation = x/C × 100, which tells you whether the approximation would have been safe.

Common questions

What is Ka?: Ka is the acid dissociation constant. For HA ⇌ H⁺ + A⁻ at equilibrium, Ka = [H⁺][A⁻]/[HA]. Smaller Ka means a weaker acid.
When can I use the approximation [H⁺] ≈ √(Ka × C)?: When ionisation is less than 5% (typically when Ka < C/100). This calculator solves the full quadratic so it works in all cases.
What is pKa?: pKa = -log₁₀ Ka. Acetic acid has Ka = 1.8 × 10⁻⁵, so pKa ≈ 4.74. The smaller the pKa, the stronger the acid.
How is this different from a strong acid?: A strong acid (HCl, HNO₃) fully ionises, so [H⁺] = initial concentration. A weak acid only partly ionises, so [H⁺] is much less and pH depends on Ka.