MCAT General Chemistry Question 293: Answer and Explanation

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Question: 293

6. What would be the pH of a solution made from combining 50 mL of 0.030 M acetic acid (Ka = 1.8 × 10- 5) and 10 mL of 0.15 M sodium acetate?

  • A. pH = 1.6
  • B. pH = 2.5
  • C. pH = 3.3
  • D. pH = 4.7

Correct Answer: D

Explanation:

D The final solution is composed of (50 mL)(0.03 M) = 1.5 mmol of HC2H3O2 and (10 mL)(0.15 M) = 1.5 mmol of NaC2H3O2 (or 1.5 mmol of C2H3O2-). The total volume will be 60 mL and the starting concentration of acetic acid will be the same as the starting concentration of its conjugate base. Since acetic acid is a weak acid, any subsequent dissociation will be relatively insignificant and the equilibrium concentrations of acid and base will remain approximately the same. When the concentration of the two species in a conjugate pair are equal, the pKa = pH from the Henderson-Hasselbalch equation: pH = pKa + log [conjugate base]/[acid]. The pKa of acetic acid (Ka = 1.8 × 10-5) is approximately 4.7.

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