MCAT General Chemistry Question 238: Answer and Explanation

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Question: 238

7. A 25.0 mL solution of 0.2 M acetic acid (pKa = 4.76) is mixed with 50 mL of 1.0 M sodium acetate (pKb = 9.24). What is the final pH?

  • A. 4.8
  • B. 5.8
  • C. 9.2
  • D. 10.2

Correct Answer: B

Explanation:

B The sodium acetate solution will be completely ionized:

NaC2 H3O2 → Na + C2H3 O2-

However, acetic acid will have negligible dissociation in solution:

HC2H3O2 H+ + C2H3O2- (Ka ≈ 1 × 10-5)

Therefore, for the combined solution, it is reasonable to assume that all of the HC2H3O2 is contributed from the acid solution, and all of the C2H3O2- is contributed from the salt solution:

(0.2 M HC2H3O2)(0.025 L) = 5 × 10-3 mol HC2H3O2

(1 M NaC2H3O2)(0.05 L) = 5 × 10-2 mol C2H3O2-

The new volume of 0.075 L cancels out when solving for the pH using the Henderson-Hasselbalch equation:

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