27. A buffer solution a. is used to control the pH of a solution b. contains at
ID: 575020 • Letter: 2
Question
27. A buffer solution
a.
is used to control the pH of a solution
b.
contains at least 100 times more of a weak acid than its conjugate base
c.
contains at least 100 times less of a weak acid than its conjugate base
d.
always has a pH of 7
28. The main intracellular buffer system is
a.
H3PO4/H2PO4
b.
H2PO4/ HPO42
c.
HPO42/PO43
d.
H3PO4/PO43
Exhibit 2B
Contains information on the pK's of some common buffers.
Buffer
pK1
pK2
pK3
Acetate
4.75
Ammonia
9.25
Carbonic acid
6.37
10.20
Citric acid
3.09
4.75
5.41
Formic Acid
3.75
Phosphoric acid
2.14
7.20
12.4
Pyruvic acid
2.50
Tris
8.3
29. Refer to Exhibit 2B. The enzyme lysozyme has an optimum pH close to 5. A suitable buffer would be:
a.
Acetate
b.
Carbonate
c.
Phosphate
d.
Pyruvate
e.
None of these is a suitable buffer for this reaction.
30. Nonphysiological buffers such as HEPES and PIPES have come into common use because
a.
they are inexpensive
b.
they can be prepared much more easily than other buffers
c.
they have less tendency to interfere with reactions
d.
they contain nitrogen
31. Which of the following is not true?
a.
A buffer is a solution which maintains a solution at a neutral pH
b.
Buffer solutions are made to resist change in pH
c.
Zwitterion buffers are less likely to interfere with biological reactions than non-zwitterions
d.
HEPES is a zwitterion buffer
32. The main blood buffer system is
a.
H2CO3/HCO3
b.
HCO3/CO32
c.
H2CO3/CO32
d.
none of the above
33. Buffers work to maintain pH because
a.
they obey LeChatlier’s principle
b.
weak acids cannot change the pH of a solution
c.
weak bases added to strong bases neutralize each other
d.
they destroy the hydrogen ion that is added
27. A buffer solution
a.
is used to control the pH of a solution
b.
contains at least 100 times more of a weak acid than its conjugate base
c.
contains at least 100 times less of a weak acid than its conjugate base
d.
always has a pH of 7
Explanation / Answer
Ans. #27. Correct option. A. Used to control the pH of a solution. Buffer is a solution that minimizes the change in pH of a solution due to addition of small amount of acid or base.
The concentration of conjugate base may either be greater than or less than the concentration of weak acid.
#28. Correct option. B. H2PO4- / HPO42
The intracellular pH is around 7.2.
So, a weak acid of pKa near the intracellular 7.2 would be most suitable as the resultant buffer has maximum buffering capacity.
Phosphoric acid dissociates as follow-
H3PO4 <---pKa 2.14--> H+ + H2PO4-
H2PO4- <-- pKa 7.20--> H+ + HPO42-
HPO42- <--pKa 12.4--> H+ + PO43-
Since pKa of H2PO4- is equal to the intracellular pH, it is the most suitable buffer.
#29. Correct option. A- Acetate.
A weak acid with pKa value closest to the desired pH forms the most suitable buffer because such buffer has maximum buffering capacity.
Among the available options, the pKa of acetic acid is closest to 5. So, it is the most suitable buffer.
#30. Correct option. C. They have less tendency to interfere with reactions.
HEPES and PIPES buffer do not yield common ions like Na+ in acetate buffer, PO43- in phosphate buffer – which can also be a part of reaction mixture. So, by not producing any undesirable common ion, these buffer have least side effects of the main reaction.
#These buffer are expensive, and need expertise for preparing them.
Presence of N- in them is not of importance here. A many compounds have N-atom as well as have buffering activity (for example, amino acids), but they can’t be used to make a buffer for carrying out reaction because they may interfere with the reaction in one or another way.
#31. A. A buffer is a solution which maintains a solution at a neutral pH. A buffer maintains the pH near the pKa value of the weak acid. It not necessarily the neutral pH. The buffer in use determine the pH being kept constant.
Option B. True. Buffer resist change in pH.
Option C. True. Because of their buffering activity, zwitter ion buffers are less likely to interfere with biological molecules when compared to non-zwitter ions without buffer activity.
# Option D. True. HEPES is a zwitterion buffer.
#32. Correct option. A. H2CO3/ HCO3-
The bicarbonate buffer system (H2CO3/ HCO3-) is the main buffer system in blood.
#33. Correct option- A. Le Chatelier’s principle states “if a dynamic equilibrium is disturbed by changing the conditions (Concentration, Volume, Pressure, temperature, etc.), the position of equilibrium shifts to counteract the change to reestablish an equilibrium”. Following the principle, a buffer neutralizes both acid and bases which are added from outside the buffer and resists the change in pH.
# Option B. Incorrect. Weak acids can change pH of a solution – the change would be proportional to the amount of acid added (to a certain extent). However, the statement is irrelevant for the given statement.
# Option C. The statement may or may not be true depending on the amount of each chemical species taken. However, the statement is irrelevant for the given statement.
Option D. Incorrect. Buffer may destroy (or, better say, neutralize) both the H+ ions and OH- ions to maintain the pH of buffer solution.
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