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 0.345 M solution of pyridine? Express the pH numerically to two decimal places.

       

Explanation:

1. Chemical equation (given)

         

2. Chemical equation with the phases:

         

3. Equilibrium constant

Only the species in aqueous phase appear in the equilibrium constant.

       

4. Calculate Kb

                     

5. Write the ICE (initial, change, equilibrium) table for the aqueous species

ICE table:

                            C₅H₅N       C₅H₅NH⁺       +OH⁻

Initial                    0.345M            0                 0

Change                   - x                 + x              + x

Equilibrium            0345 - x          x                 x

5. Substitute in the equilibrium constant

         

To solve you can neglect x in 0.345 - x, because x is much (very much) less than 0.345M.

             

6. Calculate pOH

pOH = - log [OH⁻] = - log (x) = -log (2.477 × 10⁻⁵) = 4.61

7. Calculate pH

pH + pOH = 14pH = 14 - pOH = 14 - 4.61 = 9.39

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