Weak Acid pH Calculator (from Ka or pKa)
Enter a weak acid’s concentration and Ka (or pKa) to get the pH.
Your numbers
Your results
pH
pH
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[H⁺] (mol/L)
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Percent ionised
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pH by the √(Ka·c) shortcut
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pKa
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Weak Acid pH Calculator (from Ka or pKa) by TheCalcTools
How the Weak Acid pH Calculator (from Ka or pKa) works
A weak acid HA only partly ionises: Ka = [H⁺][A⁻] ÷ [HA]. With x = [H⁺], x² ÷ (c − x) = Ka, which the calculator solves exactly with the quadratic formula.
The common shortcut x ≈ √(Ka × c) works when less than about 5% of the acid ionises. Water’s own ionisation is ignored, which is fine unless the acid is extremely dilute.
The formula
[H⁺] = (−Ka + √(Ka² + 4 Ka c)) ÷ 2; pH = −log₁₀[H⁺]
Worked example
Using the default values shown in the calculator:
| Strength given as | pKa |
|---|---|
| pKa | 4.76 |
| Ka | 1.75e-05 |
| Concentration (mol/L) | 0.1 |
- Acetic acid, pKa 4.76, 0.1 M: [H⁺] = 0.00131 mol/L.
- pH = 2.88, with 1.31% ionised.
Result:
| pH | 2.883 |
|---|---|
| [H⁺] (mol/L) | 0.00131 |
| Percent ionised | 1.31% |
| pH by the √(Ka·c) shortcut | 2.880 |
| pKa | 4.760 |
Frequently asked questions
What is the pH of 0.1 M acetic acid?
About 2.88, with Ka = 1.75 × 10⁻⁵ (pKa 4.76).
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.