🔬 pH Weak Acid Calculator
Calculate pH from Ka (acid dissociation constant) and initial concentration.
Acetic acid: 1.8×10⁻⁵
How to Use This Calculator
Enter the Ka (acid dissociation constant) for your weak acid and the initial concentration in mol/L. The calculator solves the full quadratic equation to find the exact H⁺ concentration, then returns pH, pKa, pOH, and percent dissociation.
Look up the Ka for your acid. Acetic acid is 1.8×10⁻⁵, formic acid is 1.77×10⁻⁴, hydrofluoric acid is 6.8×10⁻⁴. Enter the value in scientific notation (e.g., 1.8e-5).
Enter the initial concentration of the acid in mol/L. For a 0.1 M acetic acid solution, type 0.1.
Click Calculate pH. The result shows pH along with the equilibrium H⁺ concentration and percent dissociation.
Check the percent dissociation. If it is above 5%, the approximation method would give an inaccurate answer. This calculator always uses the exact quadratic solution.
Weak Acid pH Formula
Weak acids only partially dissociate in water. The equilibrium HA ⇌ H⁺ + A⁻ is described by Ka = [H⁺][A⁻] / [HA]. If you let x equal the amount that dissociates, you get the quadratic x² + Ka·x − Ka·C = 0. Solving for x with the positive root gives [H⁺], and taking the negative log gives pH. C is the initial concentration in mol/L and Ka is the dissociation constant for your specific acid.
Worked Examples
Where This Calculation Comes Up
Weak acid pH calculations appear throughout chemistry coursework, especially in buffer and equilibrium units. You need the pH of a weak acid to set up a Henderson-Hasselbalch calculation for a buffer solution. For example, if you are preparing an acetate buffer at pH 4.74 (the pKa of acetic acid), you first need to know the pH of the plain weak acid solution before adding any sodium acetate.
In analytical chemistry, knowing the pH of a weak acid sample matters for selecting the right indicator in a titration. If your acid has a pH of 3.5 at its initial concentration, the titration curve will look very different from a strong acid titration, and the equivalence point will be above pH 7. Food scientists calculate the pH of organic acid solutions in products like vinegar, yogurt, and citrus juices to ensure safety and taste standards are met.
Frequently Asked Questions
What is Ka?
Ka is the acid dissociation constant. It measures how completely an acid dissociates in water. A large Ka means a stronger acid; a small Ka means a weaker acid.
How is pH calculated for weak acids?
Using the equilibrium expression: HA ⇌ H⁺ + A⁻, with Ka = [H⁺][A⁻]/[HA]. Solving the quadratic gives [H⁺], then pH = −log[H⁺].
What is the pKa of acetic acid?
Acetic acid (CH₃COOH) has Ka ≈ 1.8×10⁻⁵, so pKa ≈ 4.74. A 0.1 M solution has pH ≈ 2.87.
When can I use the approximation [H⁺] ≈ √(Ka × C)?
When the percent dissociation is less than 5%, the approximation is valid. This calculator uses the exact quadratic solution for accuracy.
What does percent dissociation mean?
It is the fraction of the original acid that has dissociated, expressed as a percentage. Lower concentration and higher Ka both increase percent dissociation.