strong acid
/ STRONG AS-id /
Picture pouring a bag of sugar into water and finding that every last grain dissolves with none left at the bottom. A strong acid behaves the same way with its hydrogen ions: drop it into water and essentially all of it lets go of its proton at once, holding nothing back.
Precisely, a strong acid is one that dissociates completely, or very nearly so, in water — every molecule donates its hydrogen ion to the solvent, so the concentration of hydronium ions equals the concentration of acid you added. There is no meaningful pool of undissociated acid left, which is why a strong acid has no useful Ka value: the equilibrium lies so far toward products that the constant is enormous. A short list, including hydrochloric, nitric, sulfuric, and perchloric acids, covers the common cases.
Strong acids matter because their pH is easy to predict directly from concentration, and they give the sharpest titration jumps. The honest caveats are two: 'strong' means fully dissociated, not necessarily dangerous or concentrated, and at very low concentrations you cannot ignore the tiny extra hydrogen ions that water itself contributes through autoprotolysis.
A 0.01 mole-per-litre solution of hydrochloric acid dissociates completely, so its hydronium concentration is also 0.01 mole per litre and its pH is simply 2 — read straight off the concentration with no equilibrium math.
Complete dissociation makes hydronium concentration equal the acid concentration.
Strong is not the same as concentrated. A dilute strong acid is fully dissociated but can have a high pH, while a concentrated weak acid stays mostly intact yet can be quite corrosive — strength describes dissociation, concentration describes amount.