b.
The pK
a1
for H3PO4 is 2.22 pH and the pK
a2
for H3PO4 is 7.12 pH. The pk
a
’s for the
phosphoric acid titration curve are simply just the pH listed on the graph at the midpoint
1 & midpoint 2. The pH for midpoint 1 is found from finding the equivalence point 1 on
the graph and then taking half of that to get halfway between the start of the titration
and the equivalence point. The pk
a
for midpoint two is found the same way however it is
halfway between the first and second equivalence points.
At the midpoint, the
Henderson-Hasselbalch equation simplifies to pH=pK
a
. As stated in the lab manual the
pk
a
for each successive proton dissociation is the pH at the midpoint of the reaction for
that specific proton; in other words the midpoint is where half of those protons are
titrated. The pH for the pK
a1
listed in the book is 2.1 and the pH for my pKa1 is 2.22.The
pH for the pK
a2
listed in the book is 7.2 and the pH for my pK
a2
is 7.12. The pk
a
’s listed in
the book are very close to those that I found experimentally meaning there was little
error in the experiment because they are very similar.
c.
The K
a1
for H
3
PO
4
is 6.03x10^-3 and the K
a2
for H3PO4 is 7.6x10^-8. The K
a
’s were found
by finding the midpoint’s on the graph for the 2 equivalence points. From getting the
midpoints we were able to get the pH at the two specific points. There is a relationship
between K
a
and pK
a
. PK
a
is the pH. Then to find the K
a
you can take the negative log of
the pH. For midpoint 1 the pH was 2.22 so to find K
a
I did 10^
-2.22
. I did the same thing for
the midpoint to but with the pH for the midpoint two which was 7.12. I know that the
K
a
’s that I calculated are close to the real K
a
values because The K
a1
I found in the book for
H
3
PO
4
is 7.5x10^
-3
and the K
a2
I found is 6.2x10^
-8
. The fact that they are fairly close to
each other means that the equivalence points that I found in my experiment are fairly
close to the actual equivalence points.

d.
The H
3
PO
4
concentration in Cherry Cola is 0.0544 M. I found this molarity the same way I
found the initial concentration of H
3
PO
4
with the phosphoric acid titration curve. The first
step I took in the process of determining the molarity of H
3
PO
4
in the Cherry Cola was
finding the two equivalence points. I found the equivalence points on the graph, you can
easily find them because they occur when there is a jump in the pH. By looking at the

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- Fall '11
- HeatherRankin
- Chemistry, pH, Pk, Equivalence point, Equivalence, cherry cola