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Chemistry 1LD Practice Test

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In a buffer, if [A-] equals [HA], pH equals?
Correct Answer:
pH = pKa
Explanation:
The Henderson–Hasselbalch relation tells us pH = pKa + log([A-]/[HA]). If the conjugate base and the weak acid are present in equal amounts, their ratio is 1, and log(1) = 0. That makes pH equal to pKa. So, when [A-] = [HA], the buffer’s pH is exactly the pKa of the weak acid. This is the point where the acid and its conjugate base balance, giving the buffer its characteristic buffering capacity near that pH.

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