What is the pH of a buffer solution prepared from 0.20 mol/L ethanoic acid and 0.20 mol/L sodium ethanoate (pK_a = 4.76)?
Answer and explanation
Correct answer: B. 4.76
- Step 1: Henderson-Hasselbalch equation: pH = pK_a + log10([A^-] / [HA]). Step 2. Here [A^-] / [HA] = 0.20 / 0.20 = 1.0. Since log10(1.0) = 0, pH = pK_a + 0 = 4.76. Answer: 4.76. Why not the others: Equal concentrations of weak acid and conjugate base maximize buffer capacity at pH = pK_a.
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