Ch 1 Text 3

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Author:
DesLee26
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281526
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Ch 1 Text 3
Updated:
2014-08-31 14:33:38
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Sam
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Biochem
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Test One: pH
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  1. a.       In acid-base reactions, hydrogen ions are added to molecules or removed from them. H+ (protons) ions actually exist as __ions
                                                                  i.      The concentration of H+ ions is given in __, which is the __, where [H+] is in units of molarity.
    1.       Thus, pH 7.0 refers to a solution for which –log[H+]= __; and so log[H+]=  __ and __
    hydronium 

    pH

    –log[H+]

    7.0

    -7.0

    [H+]=10log[H+]=10-7=1.0 x 10-7
  2. The Kw for water is 1.0 x 10^-14. How do we get this number?
    Because K=[H+][OH-]/[h2O] has a value of 1.8 x 10^-16; and since the concentration of water in pure water is 55.5 M, we get a Kw of 1 x 10^-14

    And, from this, we see that [H+]=10^-14/[OH-] ad [OH]=10^-14/[H+]
  3. True or False: A double helix is sensitive to pH changes.
    True; it can form at 7, but once base causes a rise to about nine, it dissociates because the hydroxide ions react with bases in DNA base pairs to remove certain protons.
  4. Which is the most susceptible proton?
    the one bound to N-1 in a guanine base
  5. Proton dissociation for a substance HA has an equilibrium constant defined by: 

    The susceptibility of a proton to removal by reaction with a base is described by its pKa value
                                                                  i.      Ka= [H+][A-]/[HA] 

    • 1.       pKa= -log(Ka)
  6.  i.      when the pH equals the pKa, what does this mean?
    the concentration of the deprotonated form of the group or molecule is equal to the concentration of the protonated form; the deprotonation process is halfway to completion
  7. 1.       The pKa for the proton on N1 of guanine is typically __. When the pH approaches this value, the proton on N1 is __-->__
    • 9.7
    • lost
    • destabilization of DNA double helix
  8. 1.       DNA double helix is also destabilized by __
    a.       Below pH 5, what happens?

    2.       Systems have evolved to mitigate changes in pH in biological systems called __, which mitigate the pH increase caused by the addition of acid or base
    low pH.

    some of the hydrogen bond acceptors that participate in base pairing become protonated and can no longer form H bonds--> separation of double helix

    buffers
  9. a. In a buffer solution of sodium acetate, adding HCL causes a pH rapid drop, but then changes more gradually until the pH reaches 3.5. Then, it falls rapidly again. 

    Why does it decrease gradually in the middle? 
    When Hydrogen ions are added, they react with acetate ions to form acetic acid. The reaction consumes some of the added hydrogen ions so that the pH does not drop. Hydrogen ions continue reacting with acetate ions until essentially all of the acetate ion is converted to acetic acid. After this point, added protons remain in the solution and the pH drops again
  10. 1.       Analyzing the effect of the buffer in quantitative terms is possible with the __
    a.       We can calculate the ratio of the concentration of acetate ion to the concentration of acetic acid by: __
    Henderson-Hasselbalch equation

    10pH-pKa
  11. a.       A buffer functions best when?. An important buffer in biological systems is based on __, which can be deprotonated in __ to form the __
    • close to the pKa value of its acid component
    • phosphoric acid,
    • three steps 
    • phosphate ion
  12. i.      At about pH 7.4, inorganic phosphate exists primarily as a nearly equal mixture of __. The concentration of inorganic phosphate in blood is typically approximately __, providing a useful buffer against processes that produce either acid or base
    • H2PO4
    • 1 mM

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