How to use the weak acid ph calculator
Find the pH of a weak acid from Ka. Enter acid concentration, acid dissociation constant ka in the fields above; the [h⁺], ph, percent ionization are computed instantly and plotted on the chart so you can see how the answer changes.
Formula
Variables
- C — Acid concentration (mol/L)
- Ka — Acid dissociation constant Ka
Worked example
With acid concentration = 0.1 mol/L, acid dissociation constant ka = 1.8e-5, the calculator gives [h⁺] ≈ 0.00133267 mol/L; ph ≈ 2.875277; percent ionization ≈ 1.332671 %.
Frequently asked questions
What is the formula for the weak acid ph calculation?
[H⁺] (mol/L) = (-Ka+√(Ka × Ka+4 × Ka × C)) / 2. pH = -log10([[H⁺]]). Percent ionization (%) = 100 × [[H⁺]] / C. Here C is acid concentration (mol/L), Ka is acid dissociation constant ka.
What inputs do I need for the weak acid ph calculator?
You need acid concentration in mol/L, acid dissociation constant ka. Each field is pre-filled with a typical example so you can see how the result responds as you edit the numbers.
How do I use this calculator?
Type values into the input fields. Results and the chart update instantly. Use Reset to restore the example values, Copy results to paste them elsewhere, or Share to copy a link that preserves your inputs.
Is the result exact?
Calculations use standard formulas and double-precision arithmetic, then display about seven significant figures. Real-world measurements carry uncertainty, so treat results as estimates unless your inputs are exact.