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Question:

What is the percentage of pyridine (C5H5N) that forms pyridinium ion (C5H5NH+) in a 0.10 M aqueous pyridine solution (Kb for C5H5N = 1.7 × 10⁻⁹)?

0.0060

0.77

1.6

0.013

Solution:

C5H5N + H2O → C5H5NH+ + OH⁻
Kb = Cα²
α = √(Kb/C)
α = √(1.7 × 10⁻⁹ / 0.1)
α = 1.30 × 10⁻⁴
Percentage of α = 1.30 × 10⁻⁴ × 100 = 0.013%