Weak Acid pH Calculator (Ka)
Weak acid pH calculator: pH from Ka and concentration (exact quadratic solution), or Ka from a measured pH.
[H⁺] = −Ka + √(Ka² + 4KaC)2
How to use this calculator
- Choose the quantity you want under “Solve for”.
- Type the known values and pick their units: any combination works.
- The result updates as you type. Open “Show the steps” to see the substitution.
For a weak acid HA, [H⁺] = (−Ka + √(Ka² + 4Ka·C))/2. The shortcut √(Ka·C) works only when dissociation is small.
Acetic acid has Ka = 1.8 × 10⁻⁵ (pKa 4.74).
Formula
[H⁺] = −Ka + √(Ka² + 4KaC)2
| Symbol | Quantity | Units |
|---|---|---|
| Ka | Acid dissociation constant Ka | — |
| C | Acid concentration | mol/L, mmol/L, µmol/L, nmol/L, mol/m³ |
| pH | pH | — |
Worked example
Given: Ka = 1.8 × 10-5 · C = 0.1 mol/L
pH = −log[H⁺]
Result: pH = 2.87528
Frequently asked questions
pH of 0.1 M acetic acid?
About 2.88 (1.3 % dissociated).