Strong acids and bases — complete list

Everything you need to identify a strong acid or base at a glance, in one page.

What “strong” means

A strong acid or base dissociates essentially completely (>99 %) in dilute aqueous solution. For a strong acid HA:

$\mathrm{HA} + \mathrm{H_2O} \longrightarrow \mathrm{H_3O^+} + \mathrm{A^-}$

The equilibrium arrow points one way. Practical consequence: [H⁺] ≈ C_acid for pH calculations — no ICE table required, no Ka needed. Same idea for strong bases: [OH⁻] ≈ C_base (times the number of OH⁻ per formula unit).

This shortcut fails at very low concentrations (below ~10⁻⁶ M), where water autoionization matters, and at very high concentrations (> ~1 M), where activity coefficients bite.

The seven strong acids

All seven are levelled to H₃O⁺ in water — you cannot distinguish their strengths in aqueous solution, only in a less-basic solvent like acetic acid. Values are approximate aqueous pKa (Harris QCA 10e §8; Zumdahl §14).

H₂SO₄ trap. The first proton is strong; the second is only moderately strong (Ka₂ ≈ 1.0×10⁻², pKa₂ ≈ 1.99). For dilute solutions you can approximate [H⁺] ≈ 2 · C_H₂SO₄, but at high concentration or in careful analytical work you have to treat the second dissociation as an equilibrium.

Strong bases

Group 1 hydroxides — all strong, all fully dissociated in water:

Heavy Group 2 hydroxides — strong but solubility-limited:

Solubility values from CRC 97e. Mg(OH)₂ is not on this list — it is only sparingly soluble and behaves as a weak base for practical purposes (milk of magnesia).

For a strong base MOH, [OH⁻] ≈ C_base. For M(OH)₂, [OH⁻] ≈ 2 · C_base — as long as the salt is actually dissolved.

Compute pH of a strong acid

Because dissociation is complete, [H⁺] equals the analytical concentration (for a monoprotic acid) and pH = -log[H⁺]. Try it with the pH calculator.

Common student mistakes

1. Assuming HF is strong because it is a hydrohalic acid. HF is a weak acid (Ka ≈ 6.8×10⁻⁴, pKa 3.17). The strong H–X trend runs HCl < HBr < HI; HF is the exception because of its short, strong H–F bond.
2. Assuming H₃PO₄ is strong because it is a mineral acid. Phosphoric acid is triprotic and weak in every step (pKa₁ 2.15, pKa₂ 7.20, pKa₃ 12.35).
3. Treating NH₃ or “NH₄OH” as a strong base. Ammonia is a weak base (Kb ≈ 1.8×10⁻⁵). “NH₄OH” is a legacy label for aqueous ammonia — no discrete NH₄OH species exists in solution.
4. Doubling H₂SO₄ blindly in a dilute strong-acid pH calc. The second proton is not strong; the doubling shortcut is only a rough estimate.
5. Ignoring Ca(OH)₂ solubility. You cannot make a 1 M Ca(OH)₂ solution — the salt won’t dissolve. A saturated solution is only about 0.02 M.