What condition is represented by the equation pH = pKa?

The equation pH = pKa represents the condition where the concentrations of an acid and its conjugate base are equal. This relationship is derived from the Henderson-Hasselbalch equation, which describes the pH of a buffer solution. When the concentrations of the weak acid (HA) and its conjugate base (A⁻) are the same, the term involving the ratio of their concentrations simplifies, leading to the pH being equal to the pKa of the acid.

This situation is particularly significant in buffer solutions, as it demonstrates the point at which the solution has optimal capacity to resist changes in pH upon addition of acids or bases. Thus, the correct answer highlights this important principle in acid-base chemistry.

Other choices do not accurately reflect the condition described. For example, a neutral solution would not necessarily have equal concentrations of acid and conjugate base, and a strong acid would result in a significant lack of its conjugate base in a solution, thus not fulfilling the condition of equal concentrations.