Experiment 7: Comparison of Unknowns: Weak Acid Identification
Date: 10/26 and 10/28/2009
Objective:
The purpose of this experiment was to use Acid-Base titrations to identify an unknown acid.
Method:
A weak acid was obtained and dried in the oven for an hour. KHP was used to standardize the NaOH.
The amount of KHP needed was determined by calculations and three samples of KHP were weighed. 25
mL of water were used to dissolve the KHP. The pH meter was calibrated and 2-3mL increments of NaOH
were used in the titration and the pH was checked after each. Three trials were performed and the data
was graphed. Unknown B was massed (0.2g) and dissolved in 20mL ethanol in 250mL Erlenmeyer flask.
Three trials were performed using the above procedure.
Data:
Unknown Trial 1
0 3.79
3 4.09
6 4.34
9 4.54
12 4.72
15 4.9
18 5.1
21 5.39
24 6.06
26 10.19
26.5 10.32
27 10.42
27.5 10.49
28 10.55
30 10.72
32 10.82
34 10.87
36 10.95
38 10.98
Unknown Trial 2
0 3.81
3 4.13
6 4.32
9 4.52
12 4.69
15 4.88
18 5.12
21 5.4
24 6.4
26 10.34
26.5 10.45
27 10.5
27.5 10.58
28 10.62
30 10.77
32 10.87
34 10.93
36 10.98
38 11.02
Unknown Trial 3
0 3.8
3 4.07
6 4.31
9 4.52
12 4.66
15 4.88
18 5.1
21 5.41
24 6.53
26 10.35
26.5 10.45
27 10.53
27.5 10.59
28 10.63
30 10.78
32 10.87
34 10.94
36 10.99
38 11.03
KHP Trial 1
3 0.3 0 0.1
3 0.25 3 0.08333
3 0.2 6 0.06666
3 0.18 9 0.06
3 0.13 12 0.04333
3 0.2 15 0.06666
3 0.29 18 0.09666
3 0.67 21 0.22333
2 4.13 24 2.065
0.5 0.13 26 0.26
0.5 0.1 26.5 0.2
0.5 0.07 27 0.14
0.5 0.06 27.5 0.12
2 0.17 28 0.085
2 0.1 30 0.05
2 0.05 32 0.025
2 0.08 34 0.04
2 0.03 36 0.015
KHP Trial 2
3 0.3 0 0.1
3 0.25 3 0.08333
3 0.2 6 0.06666
3 0.18 9 0.06
3 0.13 12 0.04333
3 0.2 15 0.06666
3 0.29 18 0.09666
3 0.67 21 0.22333
2 4.13 24 2.065
0.5 0.13 26 0.26
0.5 0.1 26.5 0.2
0.5 0.07 27 0.14
0.5 0.06 27.5 0.12
2 0.17 28 0.085
2 0.1 30 0.05
2 0.05 32 0.025
2 0.08 34 0.04
2 0.03 36 0.015
KHP Trial 3
3 0.27 0 0.09
3 0.24 3 0.08
3 0.21 6 0.07
3 0.14 9 0.04666
3 0.22 12 0.07333
3 0.22 15 0.07333
3 0.31 18 0.10333
3 1.43 21 0.47666
2 3.82 24 1.91
0.5 0.1 26 0.2
0.5 0.08 26.5 0.16
0.5 0.06 27 0.12
0.5 0.04 27.5 0.08
2 0.15 28 0.075
2 0.09 30 0.045
2 0.07 32 0.035
2 0.05 34 0.025
2 0.04 36 0.02
Calculations:
Amount of KHP needed for a 25mL titration:
(0.1M)(0.025L) = 0.0025mol NaOH
0.0025mol x 204.23 = 0.511g KHP
Amount of Unknown B needed for a 25mL titration:
0.025L x 0.1M NaOH x x = 0.375g
Molarity of NaOH (trial1):
0.5156gKHP x x = .1001M NaOH
Molar Mass of Unknown B (trial 1):
0.025L x 0.1001M NaOH = 0.0025025 mols
= 81.08 g/mol
pKa Calculation (trial 1):
pH at ½ E.P. = pKa=4.16
Ka = 10-pKa = 10-4.16
Ka = 6.9 x10-5
Conclusions:
The purpose of this experiment was to determine an unknown acid using an Acid-Base Titration. 0.1 M
NaOH was used to titrate the weak acid. The identity of the unknown acid is N-
Nitrosophenylhydroxylamine (cupferron) because it matches our data that has a pKa of 4.16 and a Ka
of 6.9 x10-5. Based on all the data and the determination of the unknown this experiment could be
considered a success.