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The dissociation constant of a weak acid is 1 × 10⁻⁴. In order to prepare a buffer solution with a pH = 5, the [Salt]/[Acid] ratio should be

1. 1 : 10
2. 4 : 5
3. 10 : 1
4. 5 : 4

Correct answer: 10 : 1

Solution

Using the Henderson-Hasselbalch equation, pH = pKa + log([Salt]/[Acid]). Here, pKa = -log(1 × 10⁻⁴) = 4. To achieve pH = 5, log([Salt]/[Acid]) = 5 - 4 = 1, so [Salt]/[Acid] = 10. Thus, the ratio is 10:1.

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