Find the pH of the following weak acids or bases: a 0.725 M solution of pyridine (C5H5N). Kb = 1.69 x 10-9

Answer 1

C₅H₅N + H₂O ==> C₅H₅NH⁺ + OH-

Kb = [C₅H₅NH⁺ ][ OH-] / [C₅H₅N]

1.69x10^-9 = (x)(x) / 0.725 - x (ignore x in denominator as it will be small relative to 0.725)

1.69x10^-9 = x² / 0.725

x² = 1.22x10^-9

x = 3.50x10^-5 = [OH-]

pOH = -log [O-] = -log 3.50x10^-5

pOH = 4.46

pH = 14 - pOH = 14 - 4.46

pH = 9.54