chemistry help? solution?

Pyridine (C₅H₅N) reacts with H₃O⁺ (from HCl) to give C₅H₅NH⁺.
C₅H₅N(aq) + H₃O⁺(aq) → C₅H₅NH⁺(aq) + H₂O(l)
Mole ratio C₅H₅N : H₃O⁺ : C₅H₅NH⁺ = 1 : 1 : 1

Initial [C₅H₅N] = 0.020 M
Initial [H₃O⁺] = (0.0010 mol) / (200/1000 L) = 0.005 M
Obviously, H₃O⁺ is limiting reactant (completely reacts), and C₅H₅N is in excess.

After reaction, neglecting the dissociation of C₅H₅N :
[C₅H₅N]ₒ = (0.020 - 0.005) = 0.015 M
[C₅H₅NH⁺]ₒ = 0.005 M

Consider the dissociation of C₅H₅N :
C₅H₅N(aq) + H₂O(l) ⇌ C₅H₅NH⁺(aq) + OH⁻(aq) ...... Kb = 1.80 × 10⁻⁹

pOH = pKb - log([C₅H₅N]/[C₅H₅NH⁺] = -log(1.80 × 10⁻⁹) - log(0.015/0.005) = 8.27
pH = 14 - 8.27 = 5.73