A fermentation process for producing citric acid requires the broth to be maintained…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A fermentation process for producing citric acid requires the broth to be maintained at 25∘C and a pH of 4.80±0.05 for optimal enzyme activity. Two weak acid–conjugate base pairs are available in the laboratory, each at sufficient purity and cost:
• Pair X: weak acid HX, Ka=1.6×10−5
• Pair Y: weak acid HY, Ka=3.2×10−6
Assume that, for either pair, the total formal concentration of buffer components will be 0.10M and that the temperature will remain constant.
(a) Identify which acid–base pair (X or Y) is better suited to control the fermentation broth at the target pH. Support your choice with quantitative reasoning.
(b) Describe, in words, how the numerical closeness of the target pH to the acid’s pKa influences the magnitude of the buffer’s resistance to pH change when small amounts of strong acid or strong base are introduced.
(c) Suppose you prepared 1.00L of the buffer you chose in part (a) using equimolar concentrations of the conjugate acid and conjugate base. Qualitatively predict whether the pH would rise, fall, or remain unchanged after adding 5.0mL of 0.10M NaOH, and justify your answer without performing a numerical calculation.