pH Calculator
Calculate the pH and pOH of strong and weak acids and bases from concentration and Ka or Kb, with the full working including the quadratic solution.
What this calculator does
Choose a strong or weak acid or base, enter the concentration (and Ka or Kb for weak ones), and get [H⁺], pH, pOH and the percent dissociation with each step written out.
When you need it
pH problems appear in acid-base chapters, buffer design, titrations and biochemistry. In the lab, knowing the expected pH tells you whether a measurement makes sense.
Formulas
- pH = −log₁₀[H⁺]
- pOH = −log₁₀[OH⁻]
- pH + pOH = 14 (at 25 °C)
- Weak acid: Ka = x² ÷ (C − x), solve the quadratic for x = [H⁺]
How to use it
- Pick acid or base and strong or weak.
- Enter the concentration in mol/L, and Ka or Kb for a weak one.
- Read pH, pOH, the hydrogen ion concentration and, for weak acids, the percent dissociation.
Worked example
Find the pH of 0.10 M acetic acid, Ka = 1.8 × 10⁻⁵.
- Ka = x² ÷ (0.10 − x)
- Solve the quadratic: x = [H⁺] = 1.33 × 10⁻³ M
- pH = −log(1.33 × 10⁻³) = 2.88
- Percent dissociation = 1.33 × 10⁻³ ÷ 0.10 × 100 = 1.3%
Answer
pH = 2.88, and only about 1.3% of the acid has dissociated.
Common mistakes
- Treating a weak acid as strong, which gives a pH far too low.
- Forgetting that pH + pOH = 14 only holds at 25 °C.
- Using the approximation x ≪ C when the acid is more than about 5% dissociated.
Related tools
Frequently asked questions
How do you calculate pH?
pH is the negative base-10 logarithm of the hydrogen ion concentration in mol/L. For 0.0025 M HCl, a strong acid, [H⁺] = 0.0025 and pH = 2.60.
What is the difference between a strong and a weak acid?
A strong acid dissociates completely, so [H⁺] equals its concentration. A weak acid only dissociates partly and needs its Ka.
How do I get pH from pOH?
At 25 °C, pH = 14 − pOH.