Which base(s) is weaker than ammonia?

Compare the base ionization constants (Kb): ammonia has Kb ≈ 1.8 × 10⁻⁵, hydroxylamine ≈ 1 × 10⁻⁸, methylamine ≈ 4 × 10⁻⁴, and pyridine ≈ 2 × 10⁻⁹. A smaller Kb means a weaker base, so hydroxylamine and pyridine are weaker than ammonia; methylamine is stronger. The correct choice is hydroxylamine and pyridine.

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