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A buffer solution is prepared in which the concentration of NH₃ is 0.30 M and the concentration of NH₄⁺ is 0.20 M. If the equilibrium constant, Kb for NH₃ equals 1.8 × 10⁻⁵, what is the pH of this solution? (log 2 = 0.3, log 3 = 0.5).

1. 9.08
2. 9.43
3. 11.72
4. 8.73

Correct answer: 9.43

Solution

The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation for a basic buffer: pOH = pKb + log([NH₄⁺]/[NH₃]). Here, pKb = -log(Kb) = -log(1.8 × 10⁻⁵) ≈ 4.74. Substituting the concentrations, pOH = 4.74 + log(0.20/0.30) = 4.74 + log(2/3) = 4.74 - 0.18 = 4.56. Finally, pH = 14 - pOH = 14 - 4.56 = 9.43.

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