pKa from Acid Dissociation Constant

Acid strengths span more than twenty orders of magnitude, so chemists do to Ka exactly what Sørensen did to [H⁺]: take the negative logarithm. Acetic acid's Ka of 1.8 × 10⁻⁵ becomes pKa 4.74, hydrofluoric acid's 6.8 × 10⁻⁴ becomes 3.17, and formic acid's 1.8 × 10⁻⁴ becomes 3.75: three numbers you can hold in your head and compare at a glance. Lower pKa means stronger acid, and every unit is a tenfold jump in dissociation.

The sign is the perennial trip-up: because Ka is smaller than one for every weak acid, its logarithm is negative and the minus sign flips pKa positive. A pKa of 3.75 corresponds to Ka = 10⁻³·⁷⁵ = 1.78 × 10⁻⁴, not 10³·⁷⁵. pKa also does double duty as a practical marker: it is the pH at which an acid is exactly half ionised, which is why pharmacologists read a drug's pKa straight off the label to predict whether it will cross a membrane in the stomach (pH 2) or the small intestine (pH 7).