A buffer is prepared by dissolving 0.20 mol of ammonia (NH₃) and 0.10 mol of ammonium chloride (NH₄Cl) in water to make 500 mL of solution at 25 °C.
Gaseous HCl is then bubbled into the solution in small increments. Assume the volume change is negligible.
(a) Write the balanced net ionic equation for the reaction that occurs each time a small amount of HCl is introduced while the solution still behaves as a buffer.
(b) Calculate the maximum number of moles of HCl that can be added before the solution ceases to act as an effective buffer. Show your reasoning clearly.
(c) Explain mechanistically why addition of HCl beyond the amount calculated in part (b) causes a dramatic drop in pH, whereas additions up to that amount do not.