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

7. Calculate pH