Orlando 1
Quantitative Analytical Chemistry
Joseph Orlando
Lab #7: Comparison Of Unknowns: Weak Acid Identification
Purpose:
The purpose of this experiment is to introduce a deeper understanding of acid-base chemistry
using a guided inquiry lab.
Procedure:
Standardization of Strong Base
1. Prepare a 0.1M KHP solution by adding ~2 grams to a 100ml volumetric flask.
2. Prepare a 0.1M NaOH solution in a 250mL volumetric flask by dissolving 42mL of 0.6M
NaOH.
3. Calibrate pH meter using pH 4,7, & 10 standards.
Weak Acid/Strong Base Titration
1. Using a 30mL sample of prepared 0.1M KHP, titrate 2mL increments of NaOH. Record pH
after each increment.
2. Decrease increments to 0.5mL when pH rapidly changes.
3. When pH change becomes minimal, change increments back to 2mL. Record pH after each
increment. Repeat steps 1-3 two additional times.
Unknown Acid/Strong Base Titration
1. Use ~0.4g unknown acid (C) and dilute in 50mL diH2O in a 250mL Erlenmeyer flask.
2. Titrate with NaOH in the same manner as the KHP titration
Data:
KHP Standardization
Equivalence Point(mL)
Trial #1
26.60
½ Equivalence
Point(mL)
13.30
[NaOH]
Trial #2
26.70
13.35
0.0938 M
Trial #3
27.00
13.50
0.0927 M
Average
26.76
13.38
0.0935 M
0.0941 M
Orlando 2
Unknown Titration
Mass(g)
Acid used
Equivalence
Point(mL)
23.00
½
Equivalence
Point(mL)
11.50
Molecular
Weight of
Acid(g/mol)
136.56
Trial #1
0.3008
Trial #2
pKa
4.10
0.3068
24.00
12.00
132.54
4.10
Trial #3
0.3102
26.50
13.25
133.15
4.10
Average
0.3059
24.50
12.25
133.46
4.10
Ka
7.94
× 10−5
Equations:
[NaOH]
2.0464𝑔 𝐾𝐻𝑃 ×
1 𝑚𝑜𝑙 𝐾𝐻𝑃
1 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻
1
30𝑚𝐿
×
×
𝑥
= 0.09356 𝑀 𝑁𝑎𝑂𝐻
204.23 𝑔 𝐾𝐻𝑃 1 𝑚𝑜𝑙 𝐾𝐻𝑃 0.02676 𝐿 𝑡𝑖𝑡𝑟𝑎𝑛𝑡 100𝑚𝐿
Molar Mass (g/mol) of Unknown Acid
. 0245 𝐿 𝑁𝑎𝑂𝐻 ×
0.09356 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻 1 𝑚𝑜𝑙 𝑈𝑛𝑘𝑛𝑜𝑤𝑛 𝐴𝑐𝑖𝑑
×
= 0.002292 𝑚𝑜𝑙 𝑈𝑛𝑘𝑛𝑜𝑤𝑛 𝐴𝑐𝑖𝑑
1𝐿
1 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻
0.3059 𝑔 𝑈𝑛𝑘𝑛𝑜𝑤𝑛 𝑢𝑠𝑒𝑑
= 133.46 𝑔/𝑚𝑜𝑙
0.002292 𝑚𝑜𝑙 𝑈𝑛𝑘𝑛𝑜𝑤𝑛
Ka
𝐾𝑎 = 10−𝑝𝐾𝑎 = 10−4.1 = 7.94 × 10−5
Possible Identity of Unknown Acid
Ka
pKa
Benzoic Acid
Molecular
Weight(g/mol)
122.12
6.28E-5
4.202
2,4-Dinitrophenol
184.106
7.69E-5
4.114
3-Chloropropanoic Acid
105.52
7.8E-5
4.108
2,2-Bipyridine
156.15
4.6E-5
4.337
Orlando 3
KHP Standardization
14.00
13.00
12.00
11.00
10.00
9.00
pH
8.00
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
0.00
5.00
10.00
15.00
20.00
25.00
mL NaOH Titrated
30.00
35.00
40.00
Unknown Weak Acid-Strong Base Titration
13.00
12.00
11.00
10.00
9.00
pH
8.00
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
0.00
5.00
10.00
15.00
20.00
mL NaOH added
25.00
30.00
35.00
40.00
Orlando 4
Conclusion:
The purpose of this experiment was to determine the identity of an unknown acid. The
unknown acid that we used was unknown acid C. Upon completion of the titrations, we were
able to determine the equivalence point and half equivalence point of Unknown C by viewing
the titration curves. Using the information gathered from the titrations, we were able to
determine the Ka, pKa, and molecular weight of Unknown Acid C. Since the Ka (7.94E-5) and the
molecular weight (133.46 g/mol) were known, the possible identity of our unknown could be
narrowed down to several acids including benzoic acid (Ka=6.28E-5, MW=122.12) and 2,4Dinitrophenol (Ka=7.69E-5, MW=184.106 g/mol). However, since only benzoic acid's MW was
similar to our unknown's MW, it was concluded that Unknown C was in fact Benzoic Acid.
Although our data did not exactly match the data given for Benzoic Acid due to possible random
and systematic error, our experiment proved successful in determining the identity of our
unknown.