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A monoprotic acid HA dissociates according to HA ⇌ H+ + A−. If a 1.0 M solution of this acid has pH 5, what is its dissociation constant?
- 5
- 5 × 10⁻⁸
- 1 × 10⁻⁵
- 1 × 10⁻¹⁰
Correct answer: 1 × 10⁻¹⁰
Solution
The dissociation constant (Ka) can be calculated using the concentration of hydrogen ions and the initial concentration of the acid. Given that the pH is 5, the concentration of H+ ions is 1 × 10⁻⁵ M, and using the formula Ka = [H+][A−]/[HA], we find that the dissociation constant is 1 × 10⁻¹⁰.
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