Final answer:
The correct answer is a) pH; pKa. When the pH of a solution is equal to the pKa of the acid it contains, the acid is half deprotonated, meaning the concentrations of the acid and its conjugate base are equal.
Step-by-step explanation:
The Henderson-Hasselbalch equation, which is a rearrangement of the equilibrium constant expression for the dissociation of a weak acid, provides a straightforward method for calculating the pH of a buffer solution. The equation is expressed as pH = pKa + log([A-]/[HA]). Looking at the equation, we can understand that when the pH of a solution containing an acid is equal to the pKa of that acid, then the acid is half deprotonated, indicating that the concentrations of the conjugate base [A-] and the acid [HA] are equal. This is because the log of 1 (which is the result when [A-] = [HA]) is 0, and thus the pH equals the pKa at this point.