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What is [H⁺] in mol/L of a solution that is 0.20 M in CH₃COONa and 0.10 M in CH₃COOH? Ka for CH₃COOH = 1.8 × 10⁻⁵.

1. 3.5 × 10⁻⁵
2. 1.1 × 10⁻⁵
3. 1.8 × 10⁻⁵
4. 9.0 × 10⁻⁶

Correct answer: 1.1 × 10⁻⁵

Solution

This is a buffer solution of CH₃COOH and CH₃COONa. Using the Henderson-Hasselbalch equation, pH = pKa + log([A⁻]/[HA]). Substituting values: pKa = -log(1.8 × 10⁻⁵) ≈ 4.74, [A⁻] = 0.20, [HA] = 0.10. pH = 4.74 + log(0.20/0.10) = 4.74 + 0.30 = 5.04. [H⁺] = 10⁻⁵.⁰⁴ ≈ 1.1 × 10⁻⁵ M.

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