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A solution of benzoic acid is titrated with NaOH. The pH of the solution at the half-neutralisation point is 4.2. What is the dissociation constant Ka of benzoic acid?

1. 6.31 * 10⁻⁵
2. 3.2 * 10⁻⁵
3. 8.7 * 10⁻⁸
4. 6.42 * 10⁻⁴

Correct answer: 6.31 * 10⁻⁵

Solution

The Henderson-Hasselbalch equation is pH = pKa + log([A-]/[HA]). At the half-equivalence point, half the acid has been neutralised so [benzoate] = [benzoic acid], making the log term zero. Therefore pH = pKa = 4.2, and Ka = 10^(-4.2) = 6.31 * 10⁻⁵.

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