Phosphoric Acid Titration Curves
Taylor Black
Cassie Pressnall
CHEM 112L & 005
March 29, 2016
A.
Research Questions
a.
What is the exact initial concentration of the H
3
PO
4
solution?
b.
What are the pK
a1
& pK
a2
for H
3
PO
4
?
c.
What are K
a1
& K
a2
for H
3
PO
4
?
d.
What is the H
3
PO
4
concentration in my soft drink?
B.
Observations & Results
Table 1. Phosphoric Acid Titration Data and Concentration Results
[NaOH]
(M)
Equivalenc
e point
pH at
equivalenc
e point
NaOH
volume to
reach
equivalence
point (mL)
NaOH
Moles
(mol)
H
3
PO
4
Moles
(mol)
Starting
[H
3
PO
4
]
(M)
Average
[H
3
PO
4
]
(M)
0.103 M
1
st
4.33
13.0 mL
0.001339
0.001339
0.0446
0.0455
2
nd
9.48
26.5 mL
0.001391
0.001391
0.0464
Table 2. Phosphoric Acid Equilibrium Constants
Table 3. Cola Soft Drink Titration Data and Concentration Results for Cherry Cola
Graphs attached to the back
Calculations and analysis are in notebook pages
C.
Claims
Equivalence #
pK
a
K
a
1
2.22 pH
6.03x10^
-3
2
7.12 pH
7.60x10^
-8
[NaOH]
(M)
Equivalence
point
pH at
equivalence
point
NaOH
volume to
reach
equivalence
point (mL)
NaOH
Moles
(mol)
H3PO4
Moles
(mol)
Starting
[H3PO4]
in this
cola
(M)
Average
[H3PO4]
in this
cola
(M)
0.0515 M
1
st
5.52
8.0 mL
0.000412
0.000412
0.103
0.0544
2
nd
7.48
12.5 mL
0.000231
0.000231
0.00579

a.
The exact initial concentration of H
3
PO
4
is 0.0455 M.
b.
The pKa1 for H
3
PO
4
is 2.22 pH and the pKa2 for H
3
PO
4
is 7.12 pH.
c.
The K
a1
for H
3
PO
4
is 6.03x10^
-3
and the K
a2
for H
3
PO
4
is 7.6x10^
-8
.
d.
The H
3
PO
4
concentration in Cherry Cola is 0.0544 M.
D.
Evidence
a.
The exact initial concentration of H
3
PO
4
is 0.0455 M in my experiment. I found this by
using the concentration of the 0.103 M NaOH times the number of liters added to reach
each equivalence point. That gave me the number of moles of NaOH added to reach the
1
st
equivalence point.
Since the balanced chemical reaction is a 1:1 ratio that also gave
me the number of H
3
PO
4
moles titrated. Since 0.013 L of NaOH to reach the first
equivalence point and 0.0135 L of NaOH were added to reach the 2
nd
equivalence point
the two concentrations for H
3
PO
4
are slightly different. In order to get from moles to
molarity. The moles of the H
3
PO
4
had to be divided by the liters of the acid added to get
the Molarity. The concentration for the first equivalence point being 0.0446 M, and the
concentration for the second equivalence point being 0.0464 M. The average of the two
molarities of H
3
PO
4
is 0.0455.

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- HeatherRankin
- Chemistry, pH, Pk, Equivalence point, Equivalence, cherry cola