A laboratory technician prepares 1.00 L of a buffer solution by mixing 0.25 mol of acetic acid (CH₃COOH) with 0.25 mol of sodium acetate (CH₃COONa) in water. The initial temperature is 25 °C and the pH of the solution is measured to be 4.74.
Strong acid and strong base are added in separate trials, as described below. In every case, the added volume is small enough that the total volume of the buffer may be considered unchanged.
(a) Trial 1: 5.0 mL of 0.10 M HCl is added.
(b) Trial 2: 5.0 mL of 0.10 M NaOH is added.
(c) Trial 3: 5.0 mL of 0.10 M HCl is added to 1.00 L of pure water having the same initial pH as the buffer.
Answer the following questions. Provide clear reasoning in terms of the chemical species present before and after each addition.
a. For Trial 1, predict whether the pH after addition will be greater than, less than, or approximately equal to 4.74. Justify your prediction qualitatively by identifying the species that react and the products formed.
b. For Trial 2, predict whether the pH after addition will be greater than, less than, or approximately equal to 4.74. Justify your prediction qualitatively by identifying the species that react and the products formed.
c. Compare the magnitude of the pH change in Trial 1 with that in Trial 3. Explain why any difference exists, citing the relative amounts of conjugate acid–base pairs present.