Zach Bensley
Experiment 7: Comparison of Unknowns: Weak Acid Identification
Purpose:
The purpose of this experiment was to introduce the students to a deeper
understanding of acid-base chemistry, using a guided inquiry lab.
Procedure:
The way that the standardization of 0.1 M NaOH was conducted by using KHP. The
mass of the KHP was determined and the appropriate mass was around 0.510 g. The pH
meter was calibrated and titration was performed by using the appropriate amount of KHP
and 50 mL of water. Approximately 2-3 mL of NaOH increments were added to the flasks,
while the pH was being measured. Once there was a dramatic increase of pH, about 0.5 mL
increments of NaOH were added and this was conducted until the drastic increase of pH
settled down. Once the pH settled down, 2-3 mL increments of NaOH were added. This
was conducted for three trials.
Once achieved a dry sample of unknown, the titration was conducted as was in the
first part of the experiment.
Data:
KHP Titration:
Trial
Equivalence Point
(mL)
27.00
26.00
25.50
26.17
1
2
3
Average
Half Equivalence
Point (mL)
13.50
13.00
12.75
13.08
NaOH (M)
0.0925
0.0957
0.0976
0.0952
Unknown B Titration:
Trial
Mass of
Unknown
B (g)
1
2
3
Average
0.5248
0.5020
0.5158
0.5142
Volume of
NaOH
added at
Eq. Point
(mL)
39.00
39.00
35.00
37.67
Volume of
NaOH
added at
Half Eq.
Point (mL)
19.50
19.50
17.50
18.84
Molecular
Weight of
Acid
(g/mol)
pKa
Ka
141.34
135.21
154.80
143.78
3.35
3.35
3.16
3.29
4.46*10-4
4.46*10-4
6.92*10-4
5.28*10-4
Calculations:
Mass of KHP needed for 25.00 mL Titration:
0.025L *
1L
.1mol 204.23gKHP
*
*
 0.510g
1000mL 1L
1molKHP
Molarity of NaOH for Trial 1:

0.5100gKHP *
1molKHP
1molNaOH
1
*
*
 0.0925M
204.23gKHP 1molKHP 0.02700L
Molecular Weight of Unknown Acid B:

1mol _ unknown
 0.0037128mol
1molNaOH
0.5248gUknownAcid
g
 141.34
0.0037128mol
mol
0.039LNaOH * 0.0952M *
pKa for Trial 1:

pKa= pH at Half Equivalence point
3.35=3.35
Ka for Trial 1:
10-3.5=4.46*10-4
The following are graphs for the titrations for the standardization for NaOH:
14
12
10
8
6
4
2
0
Trial 1; pH vs. Volume (mL)
Series2
14
12
10
8
6
Series2
4
2
0
Trial 2: pH vs. Volume (mL) 12
14
12
10
8
6
Series2
4
2
0
Trial 3: pH vs. Volume (mL) 1
Conclusion:
In this experiment, one of our goals was to determine the unknown acid that we
received. We had unknown B and our average molecular mass was 143.78 g/mol. With
this acid, it had an average pKa value of 3.29 and an average Ka value of 5.28*10-4. A
possible candidate that could be our unknown is salicylic acid. Salicylic acid has a
molecular weight of 138.12 g/mol and has a Ka value of 1.07*10-3. Hence, even though
there are differences in numbers, there are still relatively close.