The bicarbonate buffer system and pH regulation
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The bicarbonate buffer system and pH regulation
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
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 (H2CO3 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 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:
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 (H2CO3) solution with a NaOH
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[question] => 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
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[description] => Reason for the Correct Answer:
An Arrhenius acid is a substance that when placed in water increases the hydrogen ion concentration.
A Bronsted-Lowry acid is a substance that donates a proton.
A Lewis acid is a substance that serves as an electron pair acceptor
Carbonic acid is an example of a arrhenius, Bronsted-Lowry, and Lewis acid.
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[each_answer] => A. I only
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[each_answer] => B. I and II only
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[each_answer] => C. II and III only
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[each_answer] => D. I, II, and III
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => From the titration curve provided in the passage, at which pH range will a carbonic acid solution serve as a good buffer?
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[answer] => 1
[description] => Reason for the Correct Answer:
Buffer regions are the flat segments of the curve because these are the regions where the pH changes very little when the titrant is added to the solution.
There are two buffer regions in this titration curve: point A and point C.
The range of a buffer region is pKa -1 to pKa + 1 of the flat segment’s pKa
The pKa of A is 6.3, so the range of this buffer region is 5.3 to 7.3. The pKa of C is 10.3, so the range of this buffer region is 9.3 to 11.3.
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[each_answer] => A. 5.3 to 7.3 and 9.3 to 11.3
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[each_answer] => B. 7.3 to 9.3 and 11.3 to 13.3
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[each_answer] => C. 5.3 to 7.3 and 7.3 to 9.3
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[each_answer] => D. 7.3 to 9.3 and 9.3 to 11.23
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[quiz_unique_key] => 2377279144
[question] => From the titration curve provided in the passage, at which pH does sodium bicarbonate (NaHCO₃) predominate?
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[answer] => 2
[description] => Reason for the Correct Answer:
The half-equivalence point is the point where exactly one half of the acid has been converted to its conjugate base. This represents the center point of the plateau in the titration curve.
The equivalence point is the point in the titration curve when all of the acid has been neutralized by the strong base. The conjugate base of the weak acid will predominate at this point.
At the first equivalence point, the center of the vertical segment of the titration curve, the concentration of the conjugate base of H₂CO₃ is present in the most amount.
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[each_answer] => A. 6.3
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[each_answer] => B. 8.3
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[each_answer] => C. 10.3
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[question] => What is the concentration of the H₂CO₃ solution, if 50 mL of a 1.0 M NaOH solution is needed to bring a 25 mL solution of H₂CO₃ to point D on the titration curve?
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[description] => Reason for the Correct Answer:
Rearrange the following equation: M1 x V1 =M2 x V2 to solve for M2
M1 = 1 M, V1 = 50 mL, V2 = 25 mL
M2 = (1M x 50 mL) / 25 mL = 2M
Divide the concentration obtained from the above calculation by two because carbonic acid is a diprotic acid. This will yield the concentration of the carbonic solution.
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[each_answer] => A. 1.0 M
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[each_answer] => B. 2.0 M
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[each_answer] => C. 3.0 M
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[each_answer] => D. 4.0 M
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[question] => Which of the following equations can be used to calculate the pH of the carbonic acid solution from any point along the titration curve to the left of point B?
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[answer] => 4
[description] => Reason for the Correct Answer:
Henderson-Hasselbach equation is used to calculate the pH of a buffer solution.
The generalized form of the Henderson-Hasselbach equation takes on the form pH= pKa + log [A-]/[HA]; where [A-] is the concentration of the conjugate base and [HA] is the concentration of the conjugate acid.
The pKa is equal to the pH at the half equivalence point, the point where exactly one half of the acid has been converted to its conjugate base.
pH = 6.3 + log [NaHCO₃] / [H₂CO₃] is the equation that can be used to calculate the pH of the carbonic acid solution from point along the titration curve to the left of point B.
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[each_answer] => A. H = 6.3 + log [H₂CO₃] / [NaHCO₃]
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[each_answer] => B. pH = 8.3 + log [NaHCO₃] / [H₂CO₃]
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[each_answer] => C. pH = 8.3 + log [H₂CO₃] / [NaHCO₃]
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[each_answer] => D. pH = 6.3 + log [NaHCO₃] / [H₂CO₃]
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