A 200 mL sample of a laboratory buffer contains 0.10 M NH₃ and 0.10 M NH₄Cl (pKₐ of…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A 200 mL sample of a laboratory buffer contains 0.10 M NH₃ and 0.10 M NH₄Cl (pKₐ of NH₄⁺ = 9.25).
The following sequence of operations is carried out at constant temperature:
(1) Describe qualitatively how the initial 20 mL dilution is expected to affect the pH of the buffer, if at all. Justify your answer.
(2) Immediately after the dilution, predict the initial direction of pH change produced by the subsequent 0.50 mL addition of 6.0 M HCl. Provide reasoning based on the relative amounts of NH₃ and NH₄⁺ present at that moment.
(3) Suppose instead that the sequence were reversed (acid addition first, then water). Explain qualitatively how, if at all, the final pH would differ from the original sequence, and clarify what this reveals about buffer capacity versus dilution effects.
(4) Using the concepts of capacity asymmetry and qualitative reasoning, discuss the conditions under which this buffer would "break" (lose its buffering ability) when subjected to repeated small acid additions. No numerical threshold is required, but reference the relative magnitudes of component concentrations.