Guides
How to Calculate pH — Strong Acids, Weak Acids & Bases
pH = −log₁₀[H⁺]. The key is to find [H⁺] first (for bases, find [OH⁻] then convert).
Strong acids
Strong acids ionise completely, so [H⁺] = c × number of H⁺ released per formula.
0.1 mol/L HCl → pH = −log(0.1) = 1.00
0.1 mol/L H₂SO₄ (diprotic) → [H⁺]≈0.2 → pH ≈ 0.70
Strong bases
Find [OH⁻], pOH = −log[OH⁻], then pH = 14 − pOH.
0.1 mol/L NaOH → pOH = 1 → pH = 13.0
Weak acids / bases
Weak acid: [H⁺] ≈ √(Ka·c); weak base: [OH⁻] ≈ √(Kb·c) then convert.
0.1 mol/L acetic acid (Ka=1.8×10⁻⁵) → [H⁺]≈1.34×10⁻³ → pH ≈ 2.87
Notes
- The √(Ka·c) approximation holds when c is much greater than Ka.
- Very dilute solutions need water autoionisation taken into account.
Use the pH estimator — pick a substance and enter the concentration.