BIOL 201 Lecture Notes - Lecture 2: Carboxylic Acid, Amine, Acid Dissociation Constant

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6 Feb 2016
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A"dilute"aqueous"solution"of"glycine"possesses"a"pH"="2.0."Estimate"the"fraction"of"glycine"
molecules"in"(a)$the"neutral,"uncharged"form"(b)$the"cationic"form"(c)$the"anionic"form"(d)$
the"zwitterionic"form"and"(e)$identify"the"Predominant"Species."
Using$the$H1H$equation:$$pH$–$pKa$=$log$A1/HA$
"
%""αBCOOH"/"%"αBCOOB"""""%"αBNH3+"/"%"αBNH2"
2.0"–"2.4"="log"αBCOOB"""""2.0"–"9.8"="log"αBNH2"
"""""""""""""""""""""""""""αBCOOH""""""""""""""""""""αBNH3+"
B0.4"="log""αBCOOB""""""B7.8"="log""αBNH2""
""""""αBCOOH"""""""""""""αBNH3+"
Antilog"of"B0.4"="0.4" " " " " " Antilog"of"B7.8"="0"
0.4"="αBCOOB"""""""0"="αBNH2"
""1""""""αBCOOH"""""""1""""αBNH3+"
%"αBCOOB""=""0.4"""""""x"100""=""29%""""%"αBNH2""="""0"""""x"100""="0%"
""""""""""""""""""""""1"+"0.4"""""""""""""""1"+"0"
%"αBCOOH"="""1"""""""x"100""=""71%""""%"αBNH3+""="""1"""""x"100""="100%"
""""""""""""""""""""""""1"+"0.4"""""""""""""""1"+"0"
! At"pH"2.0,"29%"of"molecules"possess"an"αBCOOB""group,"therefore"71%"possess"an"αB
COOH"group."
! Nearly"100%"of"molecules"possess"αBNH3+""group,"therefore"0%"possess"an"αBNH2"
group."
"
a. The"neutral,"uncharged"form"(or"both"groups"unionized)"="(0.71)"x"(0)"x"100"="0%"
b. The"cationic"form"(only"the"αBamino"group"ionized)"="(0.71)"x"(1)"x"100"="71%"
c. The"anionic"form"(only"the"αBcarboxyl"group"ionized)"="(0.29)"x"(0)"x"100"="0%"
d. The"zwitterionic"form"(both"groups"ionized)"="(0.29)"x"(1)"x"100"="29%"
e. The"Predominant"Species"="the"Cationic"form"="71%"
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