Lab / Acids and bases
pH Calculator (Strong and Weak Acids and Bases)
Calculate pH, pOH, and the hydrogen and hydroxide ion concentrations of a strong or weak acid or base from its concentration and Ka or Kb, or the ion concentrations from a pH.
pH Calculator (Strong and Weak Acids and Bases): pH = −log₁₀[H⁺] and pH + pOH = 14 at 25 °C. A strong acid gives all its H⁺, so 0.01 M HCl has pH 2. A weak acid only partly ionises: solving Ka = x² ÷ (C − x) for x = [H⁺], 0.1 M acetic acid with Ka = 1.8 × 10⁻⁵ has pH 2.875 and is 1.33% ionised. Weak bases are handled the same way with Kb. Runs 100% locally in your browser with zero server file uploads.
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pH = −log₁₀[H⁺] and pOH = −log₁₀[OH⁻], which add up to 14 in water at 25 °C. Strong acids and bases are taken as fully dissociated; for a weak acid the concentration of H⁺ solves Ka = x² ÷ (C − x), giving x = (−Ka + √(Ka² + 4·Ka·C)) ÷ 2, and likewise for a weak base with Kb. Very dilute solutions, below about 10⁻⁶ M, need water's own ions included, which this simple model leaves out.
Worked examples
0.001 M NaOH: pOH 3, pH 11. 0.1 M ammonia with Kb = 1.8 × 10⁻⁵: pH 11.12. A solution at pH 7.4, like blood, has [H⁺] = 3.98 × 10⁻⁸ M.
To hold a pH steady, make a buffer with the buffer calculator.
Limits of the model
The calculations use concentrations in place of activities and assume 25 °C, which is accurate for dilute solutions; at high ionic strength, measured pH differs. Below about 10⁻⁶ M, water's own ions matter and an acid's pH approaches 7.
To dilute an acid to a target concentration, use the dilution calculator.
How to use it
- Choose the kind of solution.
- Enter the concentration, and Ka or Kb for a weak acid or base.
- Read pH, pOH, [H⁺], [OH⁻], and for weak ones the share ionised.
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Frequently asked questions
Why is a weak acid's pH higher than a strong acid's?
Only a small fraction of its molecules release H⁺: 0.1 M acetic acid has a pH near 2.9, while 0.1 M hydrochloric acid has pH 1.
What about acids with two H⁺, like sulfuric acid?
Enter 2 H⁺ per formula unit to treat both as fully released, a common approximation; the second dissociation of H₂SO₄ is in fact incomplete.
Can pH be negative?
Yes, for concentrations above 1 M of a strong acid, although activity effects make the simple formula less accurate there.
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