Weak Acid pH Calculator
Solved exactly, without the x ≪ C approximation.
Work out Weak Acid pH. Solved exactly, without the x ≪ C approximation. Temperature in kelvin, handled for you.
1.8 × 10⁻⁵ for acetic acid. Enter as a decimal.
pH
2.875
1.333% ionised · pKa 4.745
This solves the quadratic x² + Ka·x − Ka·C = 0 exactly rather than assuming the ionisation is negligible against the starting concentration. That approximation is taught because it is convenient, and it fails quietly for dilute solutions or acids that are not very weak — by around 5% ionisation it is already introducing visible error, and this page avoids the question by not making the assumption.
How the Weak Acid pH Calculator works
This solves the quadratic exactly rather than assuming ionisation is negligible against the starting concentration. That approximation is taught because it is convenient, and it fails quietly for dilute solutions or acids that are not very weak.
Also known as: ka to ph calculator · percent ionization calculator · ph of a weak acid · acid dissociation calculator
Frequently asked questions
How do I find the pH of a weak acid?
Solve x² + Ka·x − Ka·C = 0 for the hydrogen ion concentration, then take the negative logarithm. For 0.1 M acetic acid with Ka 1.8 × 10⁻⁵ the pH is 2.87.
When does the usual approximation fail?
Once ionisation exceeds about 5% of the starting concentration — which happens for dilute solutions and for stronger weak acids. This page avoids the question by not making the assumption.
What is percent ionisation?
The fraction of acid molecules that have donated their proton, as a percentage. It rises as the solution is diluted, which surprises people expecting concentration and ionisation to move together.
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