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Course: MCAT > Unit 3

Lesson 1: Chemical and physical sciences practice passage questions

The role of the bicarbonate buffer system in regulating blood pH


A buffer is an aqueous solution that resists changes in pH when acids or bases are added to it. A buffer solution is typically composed of a weak acid and its conjugate base. There are three major buffer systems that are responsible for regulating blood pH: the bicarbonate buffer system, the phosphate buffer system, and the plasma protein buffer system. Of the three buffer systems, the bicarbonate buffer system is arguably the most important as it is the only one that is coupled to the respiratory system.
Carbonic acid (Hstart subscript, 2, end subscriptCOstart subscript, 3, end subscript) is a weak acid (pKa1=6.3, pKa2=10.3), and is formed when carbon dioxide combines with water in a reaction catalyzed by the enzyme carbonic anhydrase. In solution, carbonic acid is present in equilibrium with the bicarbonate ion via a simple proton transfer reaction. The equilibrium is largely controlled by the Le Châtelier's principle, which states that when stress is applied to a system in equilibrium, the reaction will shift in a direction that will reduce stress. For instance, a process that acidifies blood will be neutralized by the bicarbonate ions thus minimizing the change in pH. A process that alkalizes blood will be neutralized by the equilibrium concentration of carbonic acid. The chemical reaction describing the equilibrium between carbonic acid and bicarbonate is as follows:
COstart subscript, 2, end subscript(g) + Hstart subscript, 2, end subscriptO(l) \rightleftharpoons Hstart subscript, 2, end subscriptCOstart subscript, 3, end subscript(aq) \rightleftharpoons HCOstart subscript, 3, end subscriptstart superscript, minus, end superscript(aq) + Hstart superscript, plus, end superscript(aq)
In a titration experiment, a buret is used to administer a known concentration of NaOH to a solution of carbonic acid. The pH of the solution is measured throughout the entire titration reaction using a pH meter. A titration curve is then generated relating the change in pH with respect to the volume of NaOH added to the solution. Figure 1 represents the titration curve that was obtained during the experiment.
Figure 1: Titration curve of a carbonic acid (Hstart subscript, 2, end subscriptCOstart subscript, 3, end subscript) solution with a NaOH
It can be inferred from the passage that carbonic acid is an example of which type of acid?
I. Arrhenius
II. Bronsted-Lowry
III. Lewis
Choose 1 answer: