strong acid

Meaning: A strong acid dissociates essentially completely in water, so HA + H₂O → H₃O⁺ + A⁻ runs to completion. Its Ka is much greater than 1, and its conjugate base is too weak to take protons back from water appreciably. The usual list is HCl, HBr, HI, HNO₃, HClO₄ and H₂SO₄ (first proton only; the second is weak).

Equation: pH = −log[H₃O⁺] = −log C, for a monoprotic strong acid, where [H₃O⁺] is the hydronium concentration and C is the starting acid concentration, both in mol/L (1 mol/L = 10³ mol/m³). pH has no units.

On the MCAT: Expect pH calculations (0.010 M HCl has pH 2; a tenfold dilution raises pH by 1), comparisons of pH or conductivity with an equal-concentration weak acid, and titrations. A strong acid–strong base titration has its equivalence point at pH 7 (25 °C), since Na⁺ and Cl⁻ do not hydrolyze appreciably.

Common confusion: Strong does not mean concentrated. Strength is the extent of dissociation; concentration is the amount per volume. Dilute HCl is still a strong acid, and concentrated acetic acid is still a weak one. Also, near 10⁻⁷ M or lower, water's own H₃O⁺ matters, so the simple formula fails.