A buffer is prepared by dissolving 0.40 mol of the weak base ammonia (NH₃) and 0.60 mol of its conjugate acid NH₄⁺ in enough water to make 2.0 L of solution at 25 °C. The pH is initially 9.25.
The buffer is subsequently subjected to the following modifications in separate, independent experiments:
(a) Addition of 10.0 mL of 0.20 M NaOH (strong base)
(b) Addition of 10.0 mL of 0.20 M HNO₃ (strong acid)
(c) A temperature increase to 35 °C, which causes the value of K_b for NH₃ to increase slightly, followed by the same addition described in part (a)
Respond to each part below, using qualitative reasoning that references the composition of the buffer before and after the change.
a. For experiment (a), state whether the pH after the addition will rise, fall, or remain essentially unchanged from 9.25. Explain your reasoning.
b. For experiment (b), state whether the pH after the addition will rise, fall, or remain essentially unchanged from 9.25. Explain your reasoning.
c. For experiment (c), predict whether the buffer’s ability to resist a pH change upon the NaOH addition is enhanced, diminished, or unchanged compared with experiment (a). Provide a qualitative justification that links temperature, equilibrium position, and buffer capacity.