What is the definition of a strong acid?

Answer 1

An acid (often represented by the generic formula HA [H+A−]) is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion activity greater than in pure water, i.e. a pH less than 7.0. That approximates the modern definition of Johannes Nicolaus Brønsted and Martin Lowry, who independently defined an acid as a compound which donates a hydrogen ion (H+) to another compound (called a base). Acids are described by a pKa which is essentially a hydrogen dissociation factor. Acids, discussed here in the general form HA, are proton (H+) donors and the pKa describes the dissociation behavior of that molecule. As a consequence, each acid exists in two species, the conjugate acid (HA) and the conjugate base (A-). When the pH is equal to the pKa of the molecule, the concentration of conjugate base is equal to that of conjugate acid and the molecule behaves in an acid/base equilibrium. A strong acid situation occurs when the molecule has completely dissociated to its conjugate base. As a consequence, every acid is considered a strong acid if the pH is 2 pH units higher or more than the pKa for the acid molecule. The 2 pH units is based on statistics. The best example is that of hydrochloric acid (HCl). The pKa of HCl is around -4. Since the pH scale is between 1 and 14, HCl completely dissociates regardless of the pH. For contrast, acetic acid has a pKa of about 4.76, and would act like a strong acid at pH's equal to or above 6.76, but would be a weak acid at all other pH's.