Soper-548d0ab44b774Lab 20-2 - Lab 20 pH Titration...

Lab 20: pH Titration: Phosphoric Acid in Cola Drinks
Madison Bunker
Alec Hendrix
September 29, 2014
I did not copy this work from any other students; current students or old lab reports.

Data and Results
A. Data Tables and Plots
a. Table 1. Standardization of NaOH
b. Table 2. Initial Parameters of Titration
c. Table 3. Data for Titration of Cola

d. Plot 1. Titration Curve with Approximate First Derivative
0
2
4
6
8
10
12
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
pH ΔpH/ΔV
Volume of Titrant
pH First Derivative of the Titration Curve
Equivalence point
Equivalence point
Halfway Point
Halfway Point

e. Table 4. Determination of K
a
Values
B. Sample Calculations
a. Standardized molarity of NaOH solution
Trial 1:
.066 gKHP
204.2 g / mol
=
3.232 ∗ 10
− 4
mol
.03280 L
= .00985 M
b. Initial concentration of phosphoric acid in cola sample
First equivalence point occurs after 6.41 mL of NaOH is added, so
[
H
3
PO
4
]
=
mol NaOH at equivalence point
initial volume
=
.01 M ∗ .00641 L
.02 L
= .003205 M
c. K
a1
calculated non-graphically, from initial pH and [H
3
PO
4
]
[H
3
PO
4
]=.003205M
[H
3
O + ]=10^-2.42=.003802
*error must have occurred either when measuring the initial pH or when
calculating the first equivalence point because the [H
3
O + ] could not be greater
than the initial [H
3
PO
4
].
+ ¿
H
3
O
¿
¿
− ¿
H
2
PO
4
¿
¿
¿
K
a
= ¿
Note: This is clearly a wrong answer because K
a
cannot be negative, but for the
sake of demonstrating how to calculate the K
a1
non-graphically the methodology
is correct.