pH & pOH Calculator
Convert between pH, pOH, [H⁺] and [OH⁻], or work out the pH of a weak acid from its concentration and Ka — and the reverse.
What is pH?
pH measures how acidic a solution is: it is minus the logarithm of the hydrogen-ion concentration. Because the scale is logarithmic, each step of 1 is a tenfold change — pH 3 is ten times more acidic than pH 4.
Water always contains a little H⁺ and OH⁻, and at 25 °C their product is fixed: [H⁺][OH⁻] = 10−14. That is why pH + pOH = 14. A weak acid only partly splits into ions; its Ka tells you how much.
pH Scale
pH 3Common Substances
| Substance | pH | [H⁺] (mol/L) |
|---|---|---|
| Battery acid | 0–1 | 1 |
| Stomach acid | 1.5–3.5 | 10−2 |
| Lemon juice | 2 | 10−2 |
| Black coffee | 5 | 10−5 |
| Pure water (25 °C) | 7 | 10−7 |
| Blood | 7.35–7.45 | 4×10−8 |
| Seawater | 8.1 | 8×10−9 |
| Household ammonia | 11.6 | 3×10−12 |
| Bleach | 12.5 | 3×10−13 |
Key Relations
Undo the logarithm with a power of ten. A strong acid such as HCl splits completely, so [H⁺] simply equals its concentration.
Here x = [H⁺] = [A⁻]. The exact quadratic solution is used; the common shortcut x ≈ √(KaC) works when less than 5% dissociates.
A smaller pKa means a stronger acid. At pH = pKa exactly half of the acid is dissociated — the centre of a buffer.
Sources & Verification
- S. S. Zumdahl & S. A. Zumdahl, Chemistry, 10th ed., Cengage, 2018 — Ch. 14 (Acids and Bases) and Ch. 15 (Acid–Base Equilibria).