Question

Pyridine is a weak base that is used in the manufacture of pesticides and plastic resins. It is also a component of cigarette smoke. Pyridine ionizes in water as follows: C5H5N + H2O ⇌ C5H5NH+ + OH−

The pKb of pyridine is 8.75.
What is the pH of a 0.215 M solution of pyridine? (Assume that the temperature is 25 °C.)

Answer 1

pH = 9.29

Step-by-step explanation:

• C5H5N + H2O ↔ C5H5NH+ + OH-

∴ pKb = 8.75 = - Log Kb

⇒ Kb = 1.7783 E-9 = [OH-][C5H5NH+]/[C5H5N]

mass balance:

⇒ C C5H5N = 0.215 M = [ C5H5N ] + [C5H5NH+]

charge balance:

⇒ [OH-] = [C5H5NH+]

⇒ Kb = 1.7783 E-9 = [OH-]² / ( 0.215 - [OH-] )

⇒ [ OH-]² + 1.7783 E-9[OH-] - 3.8233 E-10 = 0

⇒ [OH-] = 1.9552 E-5 M

⇒ pOH = 4.708

∴ 14 = pH + pOH

⇒ pH = 9.29