After adding 25.0 mL of 0.100 M NaOH to 100.0 mL of 0.100 M weak acid (HA), the pH is found to be 5.90. Determine the value of K_a for the acid HA. A) 4.2 × 10^-7 B) 3.5 × 10^-9 C) 1.6 × 10^-11 D) 2.1 × 10^-5 E) none of these

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Quizplus · Accepted Answer

The pH of 5.90 indicates that the solution is in the buffer region. Using the Henderson-Hasselbalch equation, pH = pKa + log([A-]/[HA]), and knowing that 25% of the acid has been neutralized, we find pKa ≈ 6.38. Therefore, Ka = 10^-6.38 ≈ 4.2 × 10^-7.

After Adding 25