Chemistry · Acids and bases
pH calculator.
pH and pOH from the concentration of a strong acid, strong base or weak acid, or from [H+], [OH-] or a known pH.
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pH formulas
pH = -log10[H+] and pOH = -log10[OH-]. At 25 C the ion product of water is Kw = 1.0 x 10-14, so pH + pOH = 14.00.
- Strong acid: fully dissociated, so [H+] equals the acid concentration times the number of protons released. This tool also includes the water term, which only matters below about 10-6 M: 1 x 10^-8 M HCl gives pH 6.98, not 8.
- Strong base: [OH-] equals the base concentration times the hydroxides released; then pH = 14 - pOH.
- Weak acid: solve Ka = x2 / (C - x) for x = [H+] using the quadratic formula, with no small-x approximation.
Worked examples. 0.010 M HCl: [H+] = 0.010 M, pH = 2.00. 0.0010 M NaOH: pOH = 3.000, pH = 11.00. A 0.10 M weak acid with Ka = 1.0 x 10-5: [H+] = 9.95 x 10^-4 M, pH = 3.00, 0.995% ionized.
Results assume 25 C and ideal behaviour. Real, concentrated or high-ionic-strength solutions differ because activity is not concentration, and a pH meter reading on a calibrated electrode is the reference.
Frequently asked questions
How do I calculate pH from concentration?
For a strong acid, pH = -log10 of its molar concentration (times the protons per formula unit). For a strong base, find pOH the same way and use pH = 14 - pOH at 25 C.
How do I calculate the pH of a weak acid?
You need its Ka. Solve Ka = x squared / (C - x) for x, the hydrogen ion concentration, then take -log10(x). This tool solves it with the quadratic formula.
Why is the pH of very dilute HCl not above 7?
Water supplies about 1.0 x 10^-7 M of H+ itself. Including it, 1 x 10^-8 M HCl comes out slightly acidic, near pH 6.98, rather than basic.
Is neutral always pH 7?
Only at 25 C, where Kw = 1.0 x 10^-14. At other temperatures Kw changes, so this tool is for 25 C.
Sources
Checked
- OpenStax Chemistry 2e, 14.2 pH and pOH
- OpenStax Chemistry 2e, 14.3 Relative Strengths of Acids and Bases (weak-acid quadratic)
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Lab safety and scope
This is arithmetic, not professional advice. Check every result against your own procedure, the safety data sheet (SDS) for each chemical, and your lab supervisor or instructor before you prepare, mix or dispose of anything. Verify values on the reagent bottle, since purity and hydration state change the real molar mass.