1
Lab #6 Report: Experiment 18
Tuan Nguyen
Hiep Ton
Dr. Erik Woodbury
Chem 1B section MW
March 04th, 2015
I. Introduction:
2
The purpose of this experiment is to determine the dissociation constant, Ka,
for a weak acid by titration with a standardized base, and learn the use of the
Calibrate pH meter. The primary goal for this lab is to identify the unknown
acid by doing experiment and use the data collected to calculate the Ka, which
is the equilibrium constant. From that, the unknown acid will be determined
according to Table E18.2 (in Lab Manual). In this experiment, the unknown
acid is a weak acid, which does not completely dissociate. When a certain
amount of strong base (NaOH) is added to the weak acid solution, the acid
will dissociate until equilibrium is reached where the number of moles of
base added equal to the number of moles of weak acid. The pH of this
reaction will be recorded; this is also a part in the process of Ka calculation.
Measurement of the pH of the reaction will be observed by using the
Calibrate pH meter. The Titration curve will be graph by the data collected
after the experiment, which are the pH and the volume of the base added.
From the curve, identify the equivalence point and half-equivalence point to
define the Ka of [H+] by the following equation:
Ka = [H+] = 10-pH
These values will decide the identity of the unknown acid.
II. Procedure:
3
An unknown acid was weight about 0.1 mg and added into a 250mL beaker.
The beaker is added from 50.0mL to 75.0mL of water and gently well mix to dissolve
the acidic solution. Into the acidic solution beaker, 2 or 3 drops of phenolphthalein
was added as the indicator. A 10.0mL buret was filled with 0.0977M of NaOH and
clamped on a stand. Carefully, the condition bottle was removed away from the
electrode. Two small beakers, which contained acidic (red) and basic (blue)
solutions in each, were obtained to calibrate the pH by immersing the electrode into
the acidic and basic solution beakers. The two small acidic and basic solution
beakers were replaced by the original 250mL acidic solution beaker without adding
any NaOH. The well mixed acidic solution in the 250mL beaker was measured its pH
as it pH remains steady. The 250mL beaker was removed from the electrode and
added 0.5mL of NaOH into the acidic solution, waited about 1 to 2 minutes to
dissolve. Then, the recording pH process was repeated. The read and added volume
were recorded. Repeating the adding 0.5mL of NaOH process until the pH unit
changes more than 0.2 units, it is near equilibrium at which the color turned pink.
As approaching to the equilibrium, the 250mL beaker was added about one drop of
NaOH into the acidic solution, waited about 1 to 2 minutes to dissolve. The
recording pH process was repeated. After passing the equilibrium point, the adding
0.5mL of NaOH was resumed for 5 or more times. The waste was disposed by the
instructor’s instruction.
4
GRAPH:
pH & Volume NaOH
14.00
12.00
10.00
pH
8.00
6.00
pH
4.00
2.00
0.00
0.00
2.00
ANALYZED GRAPH:
4.00
6.00
8.00
10.00
Volume NaOH
12.00
14.00
16.00
5
III. Result and Discussion:
In this experiment, the titration method was used to determine the k a of an
unknown sample. According to the data and the graph from data points, the
equivalence point was pH equal 7.47 and VNaOH equal 5.75. Therefore, the
Half Equivalence point was pH equal 4.20 and VNaOH equal 2.84. According to
the calculation of determine the unknown acid base on k a , the k a value was
6.31 × 10−5 which indicated the unknown sample as benzoic acid HC7 H5 O2
(from Table E19.2 Properties of Common Weak Acid). However, According to
the calculation of determine the unknown acid base on molar weight, the
molar weight value was 178.18
g
, which indicated the unknown sample as
mol
ascorbic HC6 H7 O6 (from Table E19.2 Properties of Common Weak Acid). The
identity of the unknown acid sample was either benzoic or ascorbic. Further
tests and trials are needed to determine the correct identity of the unknown
acid sample. The possible error occurred during the data collection might be
the dissolve process when added the standardized base. The solution might
not have enough time to completely dissolve.
IV. Summary:
The pH value based on the volume added from the titrated standardized base
NaOH could be used to determine the identity of the unknown acid sample.
However, as the above results of calculation based on the collected data, the
k a value was 6.31 × 10−5 (indicated benzoic HC7 H5 O2 ) and the molar weight
value was 178.18
g
mol
indicated (ascorbic HC6 H7 O6 ). The possible error
might be the dissolve time which might not be enough for the added
standardized base to dissolve in the solution. Therefore, in order to
determine the correct unknown acid sample, more tests and trials are
required.
6
Calculation:
Equivalence: pH = 7.47 and VNaOH = 5.75 mL
Therefore:
1
5.75
VNaOH at equivalence =
= 2.84 → pH = 4.20
2
2
Identify the unknown sample based on k a :
k a = [H + ] = 10−4.20 = 6.31 × 10−5
Based on the k a → Benzoic HC7 H5 O2
Identify the unknown sample based on molar weight:
Calculate mole:
VNaOH at equivalence × MNaOH = (
5.75
) L × 0.0977 = 5.62 × 10−4 mol
1000
Calculate Molar weight:
Mass of unknown acid
0.1001
g
=
= 178.18
−4
Mole of unknown acid
5.62 × 10
mol
Based on the Mol. Wt. → Ascorbic HC6 H7 O6
7
Formal Report Feedback for Tuan and Hiep
Nariaki Yamada
Long Wang
Here are some of the suggestions about your formal report:
1. Give a unit (mL) in the volume of NaOH in the graph
2. Give units in the all of the calculation part. Some of the values
(such as moles of NaOH) are lacking units.
3. The last part of the discussion and conclusion are duplicated. I
suggest you to mention about the error once.
4. Although it’s kind of obvious, you can show the calculation of pKa
in the calculation section as well.