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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: 1.8 × 10⁻⁵

Solution

The solution is a buffer because it contains a weak acid (CH₃COOH) and its conjugate base (CH₃COONa). The concentration of H⁺ can be calculated using the Henderson-Hasselbalch equation.

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