How do organisms maintain a constant pH range?
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How do organisms maintain a constant pH range?
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[post_date] => 2025-01-14 05:09:40
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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.
Controlling the environmental pH is important for living systems as many organisms can only exist within in a narrow pH range. Human plasma, for example, must be maintained at a pH within half a pH unit of 7.4. One such mechanism that helps humans accomplish this is the phosphate buffer system inside the cytoplasm of cells. This buffer system consists of dihydrogen phosphate ions 
as hydrogen-ion donor (acid) and hydrogen phosphate ions 
as hydrogen-ion acceptor (base). These two ions are in equilibrium with each other as indicated by the chemical equation in Figure 1.
Figure 1. The phosphate buffer system
If additional hydrogen ions enter the cellular fluid, they are consumed in the reaction with 
, and the equilibrium shifts to the left. If additional hydroxide ions enter the cellular fluid, they react with 
, producing , and shifting the equilibrium to the right. This behavior allows the pH of an aqueous buffer solution to change only slightly after drops of acid or base are added. The equilibrium-constant expression for this equilibrium is shown in Figure 2.
Figure 2. The value of Ka for this equilibrium is 6.23 × 10-8 at 25°C, which yields a pKa of 7.21.
This equation can be reorganized in the form of the Henderson-Hasselbalch equation:
In order to make a buffer, a weak acid must be chosen that has a pKa value within one unit of the desired pH. This weak acid is then mixed with its conjugate base in equal concentration amounts. Table 1 lists a series of weak acids and conjugate bases, along with the pKa values for each acid.
Table 1.
[post_title] => How do organisms maintain a constant pH range?
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[question] => Which solution has the smallest ratio of conjugate base to conjugate acid?
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[answer] => 2
[description] => Reason for the Correct Answer:
Information about the buffer solution’s pH is provided in the answer choice while information about the buffer solution’s pKa values are provided in Table 1.
The Henderson-Hasselbalch equation is:
pH = pKa + log([A–]/[HA]).
Mathematically, the value of log([A-]/[HA]) decreases as the value of [A-]/[HA] decreases. Therefore the smallest ratio of conjugate base to conjugate acid should have the smallest value of log([A-]/[HA]).
We first substitute the pH and pKa values of the Henderson-Hasselbalch equation for each answer choice:
4 = 3.19 + log([A-]/[HA]) for the solution of HF and NaF
5 = 4.74 + log([A-]/[HA]) for the solution of H3CCO2H and H3CCO2Na
7.5 = 7.49 + log([A-]/[HA]) for the solution of HClO and KClO
9.5 = 9.26 + log([A-]/[HA]) for the solution of NH4Cl and NH3
Simplifying both sides of the equation:
log([A-]/[HA]) = 0.81
log([A-]/[HA]) = 0.26
log([A-]/[HA]) = 0.01
log([A-]/[HA]) = 0.24
We see that A solution of HCIO and KCIO at a pH of 7.50 will yield the smallest value of log([A-]/[HA]). Therefore, a solution of HCIO and KCIO with pH = 7.50 has the smallest ratio of conjugate base to conjugate acid.
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[answers] => Array
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[0] => Array
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[each_answer] => A.A solution of NH4Cl and NH3 with pH = 9.50
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[1] => Array
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[each_answer] => B.A solution of HClO and KClO with pH = 7.50
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[2] => Array
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[each_answer] => C.A solution of H3CCO2H and H3CCO2Na with pH = 5.50
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[3] => Array
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[each_answer] => D.A solution of HF and NaF with pH = 4.00
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => How can the amount of hydrogen phosphate ions be increased in a phosphate buffer system?
[value] => Array
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[answer] => 3
[description] => Reason for the Correct Answer:
Le Chatelier’s principle can be used to predict the effect of a change in conditions in a chemical equilibrium.
According to Figure 1, this is the phosphate buffer system:
To increase the amount of hydrogen phosphate ions, the reaction of the phosphate buffer system must shift towards the product’s side of the reaction.
Hydroxide ions will react with hydrogen ions in the solution to neutralize them.
Increasing the amount of hydroxide ions will decrease the amount of hydrogen ions, shifting the reaction towards the product’s side of the reaction due to Le Chatelier’s principle. This will increase the amount of hydrogen phosphate ions in the phosphate buffer system.
)
[answers] => Array
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[0] => Array
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[each_answer] => A.Decrease the amount of hydroxide ions entering into the cellular fluid.
)
[1] => Array
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[each_answer] => B.Decrease the amount of dihydrogen phosphate ions entering the cellular fluid.
)
[2] => Array
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[each_answer] => C.Increase the amount of hydroxide ions entering into the cellular fluid.
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[3] => Array
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[each_answer] => D.Increase the amount of hydrogen ions entering into the cellular fluid.
)
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => Bovine alkaline phosphatase is an enzyme found in cattle that must be maintained in an alkaline environment. According to Table 1, Which pair of compounds composes the best solution to use in maintaining this enzyme?
[value] => Array
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[answer] => 3
[description] => Reason for the Correct Answer:
In order to make a buffer, a weak acid and its conjugate base must be chosen such that the weak acid has a pKa value within one unit of the desired pH.
An alkaline environment is a basic environment, indicated by pH levels greater than 7.
NH4Cl is a weak acid with a pKa of 9.26, which means that it will buffer with its conjugate base, NH3, at a pH level greater than 7.
)
[answers] => Array
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[each_answer] => A.H3CCO2H and H3CCO2Na
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[each_answer] => B.H3CCOCO2H and H3CCOCO2Na
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[2] => Array
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[each_answer] => C.NH4Cl and NH3
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[each_answer] => D.HF and NaF
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[3] => Array
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[quiz_unique_key] => 2377279144
[question] => If the amount of dihydrogen phosphate ions 
outnumbers hydrogen phosphate ions 
100 : 1 in a solution, what is the pH of that solution?
[value] => Array
(
[answer] => 4
[description] => Reason for the Correct Answer:
According to Figure 2, the pKa of dihydrogen phosphate ions is 7.21.
Since the pKa of the weak acid and the ratio of weak acid to its conjugate base is known, we can use the Henderson-Hasselbalch equation pH = pKa + log([A-]/[HA]) to figure out the pH of the solution.
Substituting the pKa value as well as the ratio of conjugate base to the weak acid (1/100 which is the reciprocal of 100/1), we get:
pH = pKa + log([A-]/[HA])
pH= 7.21 + log (1/100)
pH= 7.21 – 2
pH = 5.21
Alternatively, we know that having dihydrogen phosphate ions outnumber hydrogen phosphate ions will cause the solution to be more acidic and thus have a pH lower than the pKa of 7.21. Only one of the answer choices has a value below 7.21.
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[each_answer] => A.7.21
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[each_answer] => B.9.21
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[each_answer] => C.12.1
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[each_answer] => D.5.21
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[4] => Array
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[quiz_unique_key] => 2261298308
[question] => What is the best buffer for human blood?
[value] => Array
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[answer] => 1
[description] => Reason for the Correct Answer:
According to the passage, the pH of human blood is maintained around 7.4.
In order to make a buffer, a weak acid and its conjugate base must be chosen such that the weak acid has a pKa value within one unit of the the desired pH.
To convert Ka to pKa take the –log(Ka)
Only the Ka for HClO and ClO– comes close to the desired pH of 7.4 with a pKa value of 7.49.
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