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At 25 deg C, the dissociation constant Ka of a weak monoprotic acid HA is numerically equal to the hydrolysis constant Kh of its conjugate base A-. Which of the following statement(s) is/are correct?

1. The dissociation constant of the acid HA is 10^(-7).
2. The pH of a 0.1 M aqueous solution of the acid HA is 6.0.
3. The pH of a 0.1 M aqueous solution of the conjugate base A- is 8.0.
4. The pH of an aqueous solution containing 0.1 M HA and 0.01 M HCl is 2.0.

Correct answer: The dissociation constant of the acid HA is 10^(-7).

Solution

Ka = Kh and Ka * Kh = Kw = 10⁻¹⁴ (since Kh = Kw/Ka). So Ka² = 10⁻¹⁴, Ka = 10⁻⁷. Option A is correct. For 0.1 M HA: [H+] = sqrt(10⁻⁷ * 0.1) = sqrt(10⁻⁸) = 10⁻⁴, pH = 4. Option B wrong (pH = 4 not 6). For 0.1 M A-: Kh = 10⁻⁷, [OH-] = sqrt(10⁻⁷ * 0.1) = 10⁻⁴, pOH = 4, pH = 10. Option C wrong (pH = 10 not 8). For 0.1 M HA + 0.01 M HCl: HCl dominates [H+] ≈ 0.01 = 10⁻², pH = 2. Option D correct. So A and D are correct.

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