Leah Hyde
Chem 107
Lab Section A
Identification of an Organic Acid
Purpose:
The purpose of this lab was to identify a general unknown acid. Important data
such as the weight, solubility, and melting point were used to confirm the unknown.
Tests were also preformed to identify whether the acid contained a benzene ring and if it
contained a halogen. All of these properties were used in determining what the acid was
and the molecular formation of the acid.
Procedure:
To determine the molecular weight of the unknown acid, a solution containing a
given amount of the acid was titrated using a previously standardized sodium hydroxide
solution (NaOH). To indicate when the solution was saturated, phenolphthalein was used
as the indicator. Once the molecular weight was determined, several Lewis dot structures
were drawn, utilizing the mass and considering isomers. The benzene ring and the
halogen test were preformed along with finding the melting point to aid in the
conformation of the unknown acid. After the molecular formula was obtained the data
such as weight, solubility, melting point and molecular formula in accordance with the
CRC Handbook was used to determine the correct form of the compound.
1
Leah Hyde
Chem 107
Lab Section A
Standardization of NaOH:
I
II
III
IV
V
VI
VII
VIII
Mass of
flask
and
sample
(g)
Mass of
flask (g)
g PAP
Mole
PAP
66.3565
90.6403
84.3178
89.6847
88.6691
99.0516
79.9501
91.9527
65.8503
90.1317
83.8128
89.1947
88.1821
98.5455
79.4334
91.4538
0.5062
.002479
.5086
.002491
.4798
.002349
.4900
.002399
.4870
.002385
.5061
.002479
.5167
.002531
.4989
.002443
Final
buret
reading
(mL)
Initial
buret
reading
(mL)
Base
used
(mL)
Molarity
of base
(M)
33.55
31.35
26.08
22.50
42.70
25.20
18.30
34.30
17.10
16.50
11.89
6.10
27.50
9.37
2.07
18.50
16.45
14.85
14.19
16.40
15.20
15.83
16.23
15.80
.1671
.1677
.1655
.1463
.1569
.1617
.1559
.1546
*shaded columns represent the data used
Sample calculation:
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Leah Hyde
Chem 107
Lab Section A
Determination of the weight of a carboxylic acid:
I
II
III
G-477
G-477
G-477
Unknown
number
Mass of flask 90.1560
and sample
(g)
Mass of flask 89.6560
(g)
0.5000
Mass of
sample (g)
89.7031
91.0012
89.2021
90.5032
.04992
0.4980
Final buret
reading
(mL)
30.51
29.51
29.90
Initial buret
reading
(mL)
8.61
7.65
8.15
Base used
(mL)
Moles of
base used
Weight (g)
21.90
21.86
21.76
.0034
.003405
.0033902
147.05
146.6
146.8
*shaded columns represent the data used
Sample calculation:
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Leah Hyde
Chem 107
Lab Section A
Melting point:
Fast result ( wet range)
129.0-135.0
Slow result
134.5
Interpretation of molecular weight data, C-13 nmr, and experimental
results:
After completing the titrations, interpreting the C-13 nmr spectrum, finding the
melting point, and performing the halogen and benzene ring tests, the molecular structure
of the unknown acid was able to be determined. After three successful titrations of the
unknown acid, the data showed that the acid weighed about 146.8 grams. The reason the
acid is believed to weigh 146.8 grams is because the titrations produced three similar
weights and when averaged together come out with a deviation equaling less then five
parts per thousand, meaning that it is fairly accurate.
After the weight was determined the molecular formula was narrowed down.
Using the molecular weights of Carbon (C), Hydrogen (H), Fluorine (Fl), Chlorine (Cl),
Bromine (Br), Iodine (I), Nitrogen (N), and Oxygen (O), the possibilities of the molecular
formula are narrowed down. Since the unknown is an acid, it is to contain a carboxylic
acid, or COOH group, within the molecular formula. So in determining the makeup of
the compound the weight of the COOH group is subtracted from the initial weight of the
unknown leaving the left over mass the new weight to work with. The COOH group
weighs 45.02 grams and when subtracted from 146.8 it leaves about one hundred grams
to work with. The next structure that is a possibility is the benzene ring. Not only can
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Leah Hyde
Chem 107
Lab Section A
the data through numbers show that a ring is present in the molecular makeup of the
molecule but there is also a benzene ring test that can be preformed to determine the
presence of the ring. The benzene ring test requires the unknown to be placed on a
spatula and if a very gray or black cloud of smoke appears after it has been burned in a
flame, a benzene ring is present. A black cloud appeared so knowing that the benzene
ring is present the weight of that, 77.1, is taken away from the hundred that was left.
Being that there is about 26 grams to work with after the ring and the COOH group there
are few possibilities left to try. Another indicator that can be used and that aids in the
process of determining the weight is the C-13 nmr spectrum reading. The reading that
went along with the unknown shows seven different carbons are in the molecule. The
molecule may contain more than seven carbons, but no less. The spectrum picks up on
different carbons. For example, if there are three sp3 carbons in a molecule but are all
bonded to same elements or molecules and are identical, the nmr reading will only show
one peak for those carbons. If they are not the same one peak will show up for each
special carbon. Being that there are definitely six carbons in the ring and one from the
carboxylic acid group that means there are seven carbons. But not all are different
because the ring shows symmetry. There is 26 grams left; there is only room for two
more carbons, because 2 more of anything else would make the molecular weight too
much. So if on the ring there are two carbons coming off, each with two hydrogens
attached, it leaves 7 different carbons, but the weight is still off. To figure out the
remaining carbons and other elements that will fit all the information obtained, the nmr
reading is used once again. The two remaining carbons must be double bonded with only
one hydrogen coming off each because if they had two coming off each they would be
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Leah Hyde
Chem 107
Lab Section A
sp3 and show a peak in the range of 0-50. There is no peak within that range so the
unknown is that much closer to having an identity. The unknown now is known to
contain the carboxylic acid group, a benzene ring, and a pair of double bonding carbons
connecting the two structures to form C9H8O2. This formula appears to work because
the molecular formula matches up with the previously obtained weight, but isomers still
have to be sorted, along with the boiling point, which was found to be about 134.5 *C, to
find the correct name for the unknown.
To verify the correct name of the unknown acid based on the molecular formula
and the boiling point, the CRC Handbook is used to sort through all isomers with the
same molecular formula. There are 10 isomers which all are the molecular formula
C9H8O2. All of their weights are the same, 148.16 g, solubility is similar, but they all
have different boiling and melting points. An approximate melting point was obtained
through, first finding a range where the unknown was wet, and second heating the
unknown at a rate of about 2 degrees a minute to find it exact as possible. The melting
point of the acid is about 134.5 degrees Celsius and out of the 10 isomers in the CRC
Handbook, one matched it showing the closest melting point of 133.0. It also matched up
with the solubility that the unknown had in ethanol.
So the unknown acid matched up with the weight that was found out through
titrations. Then the formula was found using that weight and the C-13 nmr spectrum
reading and the benzene test. Finally the unknown was confirmed using the CRC
Handbook and matching up with the boiling points and solubility. The unknown acid
that was found in this experiment is 3-phenyl 2-propenoic acid (E).
6
Leah Hyde
Chem 107
Lab Section A
Name of
Acid
Mol.
Formula
Molecular
Weight
Melting
Point/*C
Solubility
2,5 dimethyl
benzoic acid
3-phenyl 2propenoic acid
(E)
3 phenyl 2propenoic acid
(Z)
C9H10O2
150.18
132
C9H8O2
148.16
133
H2O 1; EtOH
3; eth 3; lig2
H2O 1; EtOH
4; eth 3; ace 3
C9H8O2
148.16
42
EtOH 4; HOAc
4; lig 4
*shaded one is the unknown acid
-The dimethyl benzoic acid was considered because
the weight came close to the weight that was
obtained through the titrations and the solubility and
the melting point were close but eliminated after
figuring out that it did not agree with the C-13 nmr
results.
-The e form of the 3 phenyl 2 propenoic acid was
the correct acid because it matched up with the
molecular weight, melting point, solubility, and
agreed with the C-13 nmr results.
-The z form of the 3 phenyl 2 propenoic acid was
considered because the C-13 nmr results, weight
and the solubility were accurate but immediately
discarded when the melting point was too low.
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