At what point is a weak acid 50% protonated and 50% deprotonated?

When the pH of a solution equals the pKa of a weak acid, the acid is at the point of 50% protonation and 50% deprotonation. This concept is based on the Henderson-Hasselbalch equation, which relates the pH of a solution to the pKa of the acid and the concentrations of its protonated and deprotonated forms.

At this equilibrium point, the concentrations of the protonated form (HA) and deprotonated form (A-) are equal, meaning that half of the acid molecules are in the protonated state while the other half are in the deprotonated state. This balance is crucial for understanding the behavior of weak acids in different pH environments and is a foundational concept in acid-base chemistry.

Other options do not accurately represent when a weak acid achieves this balance. For instance, having a pKa of zero does not correlate to 50% protonation and does not indicate a condition relevant to the acid's behavior in solution. Similarly, stating that the acid is 50% protonated when the acid dissociation constant (Ka) equals 1 does not directly relate to the equilibrium point described by the pH and pKa. Lastly, a