In a buffer solution, if [A−] = [HA], which of the following must be true?

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BlackTom AI · Accepted Answer

When evaluating a buffer consisting of a weak acid HA and its conjugate base A−, the Henderson-Hasselbalch equation is the guiding principle: pH = pKa + log([A−]/[HA]). Option 1: pH = pKa. If [A−] equals [HA], then the ratio [A−]/[HA] equals 1, and log(1) = 0, so pH = pKa. This is exactly what the equation predicts for equal concentrations of acid and conjugate base. Option 2: pH > pKa. This would require [A−]/[HA] > 1 (more base than acid). Since the condition given is [A−] = [HA], this ratio is 1, not greater, so pH cannot exceed pKa under these circumstances. Option 3: pH < 7. While some buffers have pH below 7, the equality [A−] = [HA] only fixes the relation between pH and pKa, not the absolute acid strength or the solution’s ion product. The pH could be above or below 7 depending on pKa, so this statement is not compelled by the given condition. Option 4: pH > 7. Similar to Option 3, this would depend on the actual pKa value and the system’s context. The equal concentrations do not guarantee a basic (pH > 7) buffer; they only fix pH relative to pKa. Option 5: pH < pKa. This would imply [A−]/[HA] < 1. But the given condition is [A−] = [HA], which yields a ratio of 1, not less than 1, so this statement is inconsistent with the Henderson-Hasselbalch relation here. In summary, the equality of the buffer components directly leads to pH equaling pKa via the Henderson-Hasselbalch equation, while all other statements are not implied by the given condition.

CHM-1020-LD01.2025SU Quiz 5-Buffers

In a buffer solution, if [A−] = [HA], which of the following must be true?

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