ChemBench
pH Calculator
How acidic or basic is this solution?
pH and pOH for hydrogen ion concentrations
pH is the negative base-10 logarithm of the hydrogen ion concentration. Each whole pH unit is a tenfold change in acidity, and pH plus pOH always equals 14 at 25 C.
| [H+] mol/L | pH | pOH | Roughly |
|---|---|---|---|
| 1 x 10^-1 | 1.00 | 13.00 | Stomach acid |
| 1 x 10^-2 | 2.00 | 12.00 | Lemon juice |
| 1 x 10^-3 | 3.00 | 11.00 | Vinegar |
| 1 x 10^-4 | 4.00 | 10.00 | Tomato juice |
| 1 x 10^-5 | 5.00 | 9.00 | Black coffee |
| 1 x 10^-7 | 7.00 | 7.00 | Pure water, neutral |
| 1 x 10^-9 | 9.00 | 5.00 | Baking soda solution |
| 1 x 10^-11 | 11.00 | 3.00 | Ammonia cleaner |
| 1 x 10^-13 | 13.00 | 1.00 | Drain cleaner |
The pH + pOH = 14 relationship holds at 25 C. Water's ionisation constant changes with temperature, so neutral pH is about 6.14 at 100 C, not 7.
A logarithmic scale for acidity
pH = −log₁₀[H⁺] compresses a huge range of hydrogen ion concentrations (spanning many powers of 10) into a simple 0-14 scale — each single pH unit represents a 10× change in acidity.
Why it's everywhere
pH governs which chemical reactions can occur in blood, soil, swimming pools, and industrial processes — human blood, for instance, is tightly regulated between pH 7.35 and 7.45, and even small deviations are medically dangerous.
Frequently asked questions
My hydrogen ion concentration is 3.2 × 10⁻⁴ M — what is the pH?
pH = -log10(3.2 × 10⁻⁴) = 3.49. That is moderately acidic, similar to orange juice.
How is pH related to pOH?
pH + pOH = 14 at 25°C. So a pH of 3.49 means pOH = 10.51. This relationship comes from the water autoionization constant Kw = 10⁻¹⁴.
How is the pH calculator different from the Henderson-Hasselbalch calculator?
This calculator converts a known [H+] into pH. The Henderson-Hasselbalch calculator predicts a buffer solution's pH from pKa and the acid/base concentration ratio — use it when you are designing or analyzing a buffer system.
Related Science calculators
Henderson-Hasselbalch Calculator
Buffer solution pH from pKa and the acid/base concentration ratio.
OpenIdeal Gas Law Calculator
Solve PV = nRT for pressure given volume, moles, and temperature.
OpenBoyle's Law Calculator
Final pressure from initial pressure/volume and a new volume, at constant temperature.
OpenCharles's Law Calculator
Final volume from initial volume/temperature and a new temperature, at constant pressure.
OpenLast updated: September 7, 2026