MCAT General Chemistry Question 293: Answer and Explanation
Home > MCAT Test > MCAT general chemistry practice tests
Test Information
- Use your browser's back button to return to your test results.
- Do more MCAT general chemistry practice tests.
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.