CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Three monoprotic acids—benzoic acid (pK_a = 4.20), chloroacetic acid (pK_a = 2.85), and hypobromous acid (pK_a = 8.69)—are each dissolved separately to make 0.10 M aqueous solutions. Two buffered environments are prepared: Buffer X has a measured pH of 3.00, while Buffer Y has a measured pH of 6.00.
For each acid, the chemist wishes to know which conjugate form (HA or A⁻) will constitute more than 50 % of the total acid–base pair in each buffer.
(1) For the pH 3.00 environment (Buffer X), identify whether the protonated form (HA) or the deprotonated form (A⁻) will be present in greater concentration for each of the three acids. Justify your answer qualitatively without performing numerical calculations.
(2) Repeat part (a) for the pH 6.00 environment (Buffer Y), again providing qualitative justification.
(3) Choose one of the three acids and explain, in complete sentences, how comparing pH and pK_a allows you to predict the predominant form in any aqueous environment.
(4) Rank the three acids from the one that requires the lowest solution pH to be predominantly deprotonated to the one that requires the highest pH. Provide a brief explanation for your ranking.