27. A buffer solution a. is used to control the pH of a solution b. contains at
ID: 575019 • 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
3) The absorbance curve is prepared by plotting the absorbances of a series of standard solutions against their concentrations. The standard solutions are so prepared that it covers a range of concentration values starting from the lowest concentration to the highest concentration.
Now, the absorbance of the unknown solution is obtained to be greater than the absorbance of the highest standard solution. This simply means that the unknown solution has a higher concentration than the highest standard solution and hence, the calibration curve cannot be used to accurately obtain the concentration of the unknown. In this case, the unknown solution must be diluted by adding a known amount of the diluent so that the concentration of this dilute solution falls within the range of the calibration curve and we can accurately determine the concentration of the unknown. The obtained concentration must be multiplied by the dilution factor to obtain the actual concentration.
4) The absorbance of the unknown solution is obtained to be lower than the absorbance of the lowest standard solution. This simply means that the unknown solution has a lower concentration than the lowest standard solution and hence, the calibration curve cannot be used to accurately obtain the concentration of the unknown. In this case, the unknown solution must be made concentrated by adding a known amount of the sample to the unknown solution so that the concentration of this prepared solution falls within the range of the calibration curve and we can accurately determine the concentration of the unknown.
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