of weak acid HA () is mixed with of NaOH. The mixture is diluted with of water, and then millimoles of HCl are added. The pH is (use )

Answer and explanation

Correct answer: D. 4.30

  1. Step 1: Calculate the millimoles of weak acid and strong base initially present: and . Neutralization consumes of to produce of , leaving of unreacted .
  2. Step 2: Dilution with water increases the total volume to but preserves the molar ratio of to , so the buffer ratio remains unchanged. Next, adding of strong acid converts of conjugate base into : and .
  3. Step 3: Apply the Henderson-Hasselbalch equation: . Since , .
Answer
4.30.
Why not the others: is the buffer pH prior to adding , and inverts the conjugate base-to-acid ratio.

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