An enzyme assay must be carried out at pH 7.40 and produces up to 0.005 mol of…
CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases
Problem
An enzyme assay must be carried out at pH 7.40 and produces up to 0.005 mol of hydronium ion per liter during the course of the reaction. You have available the conjugate acid–base pairs listed below (all Ka values are at 25 °C):
• H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻ Ka=6.2×10−8
• HCO₃⁻ ⇌ H⁺ + CO₃²⁻ Ka=4.7×10−11
• CH₃COOH ⇌ H⁺ + CH₃COO⁻ Ka=1.8×10−5
Design a buffer solution that meets the pH requirement and is capable of neutralizing the stated amount of acid without the pH dropping below 7.20.
(a) Select the most appropriate conjugate acid–base pair from the list and justify your choice quantitatively with respect to the target pH.
(b) Determine the minimum concentrations (in mol L⁻¹) of the conjugate acid and its conjugate base that must be present to neutralize 0.005 mol of added hydronium ion per liter while keeping the pH ≥ 7.20. Show all reasoning, including any assumptions made.
(c) Write balanced net ionic equations that illustrate how your chosen buffer components react with (i) added hydronium ion and (ii) added hydroxide ion.
(d) Explain, with reference to dynamic equilibrium, why the buffer prepared in parts (a) and (b) maintains pH within the desired range during the assay, whereas a solution containing only the conjugate base at the same initial pH would not.