The Charge Balance Calculation Method and Buffers

In the previous calculations we used e (the extent of the reaction) as a calculation tool. However in aqueous chemistry calculations [4] a different calculation method, charge balance, is more convenient and widely used. We introduce it here and illustrate its use with examples of buffer solutions. 

Example 13.4 To show the effects of a buffer solution, we begin with asking the pH at 25 °C of an aqueous solution of 0.1 molal acetic acid (HAc, where Ac stands for the acetate ion, CHaCOO ). HAc is a weak acid, whose ionization (HAc) [H ] + [Ac ] at 25 °C is described by 

For the rest of this section we drop the [1 molal] denominators, remembering that concentrations are all expressed in molality (practically equal to molarity for dilute solutions). Instead of using the extent of reaction, we ask what species will be present at equilibrium, excluding H2O, which is present in excess, finding, for this reaction the species H, Ac , HAc and OH. By electroneutrality, we can write 

Where is the ionization constant for water = 10 at room temperature. By conservation of Ac we can say that 

Where (HAc)o is the amount of HAc introduced into the solution. Combining this with Eq. 13.U 

---