What is the pH of 0.75M solution of C5H5 NHCl? Kb for C5H5N = 1.7*10^ -9

Question

What is the pH of 0.75M solution of C5H5 NHCl?  Kb for C5H5N = 1.7*10^ -9

Answer 1

Since C₅H₅NHCl is a salt of a weak base (C₅H₅N) and a strong acid (HCl), the pH will be acidic.

C₅H₅NH⁺ + H₂O ==> C₅H₅N + H₃O⁺

Here the C₅H₅NH⁺ is acting as an acid, so we need to use the Ka for C₅H₅NH⁺

Ka = 1x10^-14 / 1.7x10^-9 = 5.88x10^-6

Ka = 5.88x10^-6 = [C₅H₅N][H₃O⁺] / [C₅H₅NH⁺]

5.88x10^-6 = (x)(x) / 0.75

x² = 4.4x10^-6

x = 2.1x10^-3 M = [H₃O⁺]

pH = -log [H₃O⁺]

pH = 2.68