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A weak acid HA has a pKa of 4.5. What is the pH of an aqueous buffer containing HA when 50% of the acid molecules are ionized?
- 7.0
- 4.5
- 2.5
- 9.5
Correct answer: 4.5
Solution
By Henderson-Hasselbalch, pH = pKa + log([A-]/[HA]). At 50% ionization [A-] = [HA], so log term = 0 and pH = pKa = 4.5.
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