What this calculator does
Weak acid/base pH from Ka or Kb works out pH of a weak acid or base solution from its dissociation constant and starting concentration. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaICE table: Ka = x²/(C₀−x) → x = (−Ka + √(Ka²+4·Ka·C₀)) / 2, pH = −log₁₀[H⁺]
The inputs explained
| Field | What to enter |
|---|---|
| Type | Choose from Weak acid (Ka), Weak base (Kb). |
| Ka or Kb | A number. Starts at 0.000018. |
| Initial concentration C₀ (mol/L) | A number, measured in mol/L. Starts at 0.1. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with ka or kb
Every other input is held at the calculator’s starting values while ka or kb varies. Select any row to load that scenario into the calculator.