CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases
Problem
Glycine, NH₃⁺–CH₂–COOH, is a diprotic species with pK_a1 = 2.34 (carboxyl group) and pK_a2 = 9.60 (amino group). A biochemist investigates the charge state of glycine under different physiological conditions.
(1) Derive an expression for the isoelectric point (pI) of glycine in terms of its two pK_a values, and calculate the numerical value of pI to two decimal places.
(2) For each of the following pH values—1.0, 6.0, and 12.0—determine which protonated/deprotonated form of glycine is present in greatest concentration. Support your answer with qualitative or quantitative reasoning as appropriate.
(3) At pH 6.0 the biochemist adds 0.010 mol of solid glycine to 1.00 L of a buffer composed of 0.15 M glycine in its predominant form at that pH. Predict whether the pH will increase, decrease, or remain unchanged after dissolution, and justify your prediction using acid–base principles.
(4) Design a buffer using glycine that will resist pH changes near pH 9.6. Specify the ratio of conjugate base to conjugate acid required and explain why this ratio is effective in that pH region.