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A 2.5 mL sample of a weak monobasic base of concentration (2/5) M, having Kb = 1 × 10⁻¹² at 25°C, is titrated with (2/15) M HCl in aqueous medium at 25°C. At the equivalence point, the hydrogen ion concentration is (Kw = 1 × 10⁻¹⁴ at 25°C):

1. 3.7 × 10⁻¹⁴ M
2. 3.2 × 10⁻⁷ M
3. 3.2 × 10⁻² M
4. 2.7 × 10⁻² M

Correct answer: 3.2 × 10⁻² M

Solution

Equivalence salt concentration = 1 mmol / 10 mL = 0.1 M; Ka(BH+) = 1e-14/1e-12 = 1e-2; [H+] = sqrt(1e-2 * 0.1) = 3.2e-2 M, not 3.2e-7.

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