pH Calculator
Convert between pH, pOH and ion concentrations.
Free chemistry formula cheat sheet (PDF)
One page with the formulas you use most — molar mass, grams ⇄ moles, pH and PV = nRT — each with a worked example. Leave your email and we'll send it over.
Worked examples
[H₃O⁺] = 0.005 mol/L. pH = −log₁₀(0.005) = 2.30 — strongly acidic, which is why your stomach lining needs protective mucus.
Blood has pH 7.40. Rearranging: [H₃O⁺] = 10^(−7.40) ≈ 4.0 × 10⁻⁸ mol/L. Just slightly above 7 — blood is mildly basic, and your body works hard to keep it in the narrow 7.35–7.45 range.
[OH⁻] = 0.002 mol/L. First pOH = −log₁₀(0.002) = 2.70, then pH = 14 − 2.70 = 11.30 — basic, as expected for ammonia.
How it works
pH is defined as pH = −log₁₀[H₃O⁺]: each unit on the scale is a tenfold change in acidity, which is why lemon juice (pH 2) is 100,000 times more acidic than blood (pH 7.4). pOH works the same way for hydroxide ions, and at 25 °C the two always add to 14 because [H₃O⁺] × [OH⁻] = 1.0 × 10⁻¹⁴ (the water auto-ionization constant, Kw).
Values below 7 are acidic, 7 is neutral (pure water), and above 7 is basic. Real-world pH also depends on temperature, which shifts Kw slightly — the 14-sum rule is exact only at 25 °C.
Take it with you: download the one-page formula cheat sheet (PDF) — print it and keep it on your desk.
Next steps: study titrations and neutralization in the Acids, Bases & pH lesson, or explore the periodic table to see why acids behave the way they do.