A buffer solution is prepared by dissolving 0.250 mol of NH3(aq) and 0.250 mol of…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A buffer solution is prepared by dissolving 0.250 mol of NH3(aq) and 0.250 mol of NH4Cl(s) in enough water to make 1.00 L of solution at 25 °C. The base-dissociation constant for NH3 is Kb=1.8×10−5. The buffer is subjected to two sequential dilutions while kept in a closed container at constant temperature.
Step 1: 500 mL of distilled water is added to the buffer.
Step 2: An additional 500 mL of distilled water is added to the mixture from Step 1.
Assume volumes are strictly additive and gases do not escape or dissolve appreciably.
(a) For the initial 1.00 L buffer, calculate the pH using an appropriate method.
(b) After Step 1, calculate the new pH of the solution once equilibrium is re-established. Show that any simplifying assumptions you make are justified.
(c) Identify the direction in which the equilibrium shifts during Step 1 and explain your reasoning in terms of changes in concentration of all dissolved species.
(d) Repeat the calculations in part (b) for Step 2, determining the pH after the second dilution has reached equilibrium.
(e) Discuss how the numerical results from parts (a), (b), and (d) illustrate the limits of the buffer’s capacity under successive dilutions. Include in your discussion an explanation of why dilution eventually diminishes the buffer’s ability to resist pH changes.