The bicarbonate buffering system and titration curves
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- The bicarbonate buffering system and titration curves
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The bicarbonate buffering system and titration curves
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[post_date] => 2025-01-14 05:12:03
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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.
The pH of blood in humans is around 7.4. A rise of pH above 7.45 leads to the condition of alkalosis that disrupt enzymes, causing muscle spasms and respiratory paralysis. Likewise, if physiological pH drops below 7.35, it leads to acidosis that causes depression of the central nervous system. Several factors, including exercise, diet and changes in respiratory patterns, alter physiological pH. The body responds to these changes through the action of buffers that resist the alteration of pH. The bicarbonate buffering system in human blood, for example, maintains the pH around 7.4 and is composed of carbonic acid (H2CO3) and bicarbonate ion .The buffer is formed when carbon dioxide dissolves into blood, forming carbonic acid. Carbonic acid and water then react to form hydronium ion (H3O+) and the bicarbonate conjugate base in solution.
Buffers can also be created in the laboratory by partially titrating either a weak acid with a strong base, or a weak base with a strong acid. This is visualized in a titration curve (Figure 1). After a sharp increase at the beginning, the pH during the course of the titration increases gradually due to the buffering capabilities of the solution. This continues until the base overcomes the buffers capacity after the equivalence point, and all of the initial acid has been converted to its conjugate base with the addition of the strong base.
Figure 1. The titration of a weak acid with strong base. The titration curve is a graph of the volume of titrant, or in our case the volume of strong base, plotted against the pH of the solution.
Attribution: J.A. Freyre, CC-BY-SA 2.5
The addition of an acid or a base to a buffered solution creates a smaller pH change than would occur if the acid or base were added to water alone. This behavior is described quantitatively by the Henderson-Hasselbalch equation (Figure 2), which can be used to deduce that pH of a buffer solution is equal to the pKa of the acid when equal molar concentrations of acid and its conjugate base is present. This point is also known as the half-equivalence point.
Figure 2. The Henderson-Hasselbalch equation
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[question] => What occurs when CO2 enters the blood?
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[description] => Reason for the Correct Answer:
The pKa is a quantitative measure of a weak acid describing its propensity to dissociate into its conjugate base and hydronium ion. The pKa does not change unless a change in temperature occurs.
According to the passage, when carbon dioxide dissolves into blood, carbonic acid is formed.
Carbonic acid will dissociate into (H3O+) and bicarbonate 
, thus increasing both products in the blood stream.
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[answers] => Array
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[0] => Array
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[each_answer] => A.The 
in the blood increases.
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[1] => Array
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[each_answer] => B.The pKa of the blood decreases.
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[2] => Array
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[each_answer] => C.The pH of the blood increases.
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[3] => Array
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[each_answer] => D.The [H3O+] in the blood decreases.
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => Which of the following is the conjugate base of bicarbonate?
[value] => Array
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[answer] => 4
[description] => Reason for the Correct Answer:
A conjugate base is the species formed by the removal of a proton (H+) from its conjugate acid.
Bicarbonate 
is the conjugate acid.
Bicarbonate, which has a -1 charge, with one proton removed would yield carbonate (
) , which has a -2 charge.
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[each_answer] => A.OH–
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[1] => Array
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[each_answer] => B.H2CO3
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[each_answer] => C.
)
[3] => Array
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[each_answer] => D.
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => Before performing the titration represented by the graph in Figure 1, a chemist will add a drop of another weak acid that has a pKa of around 8. What is the likely reason for the addition of this weak acid?
[value] => Array
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[answer] => 4
[description] => Reason for the Correct Answer:
A drop of weak acid will not significantly affect the acidity or basicity of a solution.
According to Figure 1, the titration graph indicates that at a pH level of 8, the equivalence point of the titration is reached. This is the point where the number of moles of added base is equal to the number of moles of the acid in the solution.
A substance that changes color in response to a change in pH levels is an acid-base indicator. Typically added at the start of a titration, a drop of indicator is specifically chosen so that its color change region, near its pKa will approximate the equivalence point of a titration experiment
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[answers] => Array
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[0] => Array
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[each_answer] => A.The additional acid provides a known starting point to to calibrate the pH of the horizontal portion of the titration graph.
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[1] => Array
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[each_answer] => B.The additional acid provides a source of hydronium ions to eliminate the excess hydroxide ions initially in the water.
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[each_answer] => C.The additional acid maintains the buffering capabilities of the solution against the strong base titrant.
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[each_answer] => D.The additional acid helps the chemist indicate when the solution passes a certain pH threshold.
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[quiz_unique_key] => 2377279144
[question] => Acetic acid has a pKa of 4.8. What is the pH of a solution containing ten times the concentration of acetic acid as sodium acetate?
[value] => Array
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[answer] => 1
[description] => Reason for the Correct Answer:
The Henderson-Hasselbalch equation relates the weak acid’s pKa the pH level of the solution, the concentration of the weak acid and the concentration of the weak base in a single equation.
A solution containing ten times the concentration of acetic acid as sodium acetate will yield a
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[each_answer] => A.3.8
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[question] => Cysteine is a crucial amino acid in the structural support of enzymes. At physiological pH, what is the net charge of the amino acid cysteine?
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[answer] => 2
[description] => Reason for the Correct Answer:
According to the passage, physiological pH is 7.4.
Amino acids are polyprotic acids with multiple protons, each with their individual pKa values.
When the pH of the solution is greater than the pKa for a proton, the proton is removed and the compound becomes deprotonated at that region. When the pH of the solution is less than the pKa for a proton, the proton remains at that region.
Cysteine has three regions with protons. The carboxylic acid functional group has a pKa (1.71) lower than physiological pH; this region is deprotonated thus carrying a -1 charge. Two of these regions, the thiol and the amine functional groups, have pKa values higher than physiological pH (8.33 and 10.78, respectively); these region are protonated thus carrying a 0 and a +1 charge respectively. The structure of cysteine at a pH of 7.4 is shown here:
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