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POGIL LAB 12 Page 1 of 6
POGIL LAB EXERCISE 12
How Do Chemists Use Stoichiometry?
Each member should assume his or her role at this time. The new manager takes
charge of the POGIL folder and hands out the GRF and RRF to the appropriate
members. The new recorder should record the names of the group members on the
new GRF and the new recorder should record the start time for the lab.
Table 1. Group Member Role Assignments
GROUP TYPE ->
GROUPS OF THREE
MEMBER NO. ->
1
2
3
Manager
+
Reporter
+
Recorder
+
Reflector
+
Technician
Encourager
+
SFUC
*
+
GROUPS OF FOUR
1
2
3
+
4
+
+
+
+
+
+
*
Managers should get the group started studying Observation I as soon as possible.
OBSERVATION I: You have been introduced to the concepts of stoichiometry in class
and now you will actually practice it in this lab period. We will be using the reaction
of sodium bicarbonate (hydrogen carbonate) and an acid which always yields three
products: 1) a salt (the cation partner to bicarbonate and the anion of the acid), 2)
water and 3) carbon dioxide. Most of the time the acid is added in excess making
the bicarbonate the limiting reagent.
The specific reactants chosen for this exercise are sodium bicarbonate and acetic
acid (HOOCCH3). The reason for using this specific example is simple: the only
significant product left in the reaction is the salt because the carbon dioxide escapes
into the atmosphere and the water formed vanishes in the aqueous environment.
Finally, the excess acetic acid left in the mixture is a liquid that has a boiling point
slightly less than water. Thus, the result of the reaction is a salt solution plus excess
acetic acid – a mixture that can easily be separated by evaporation.
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1. Write a balanced overall equation for the reaction of acetic acid and sodium
bicarbonate.
2. Write a net, balanced ionic equation for this reaction.
3. State two reasons why you should know that an actual reaction takes place:
a.
b.
Recorders should contact the instructor to validate the group’s work to this point.
OBSERVATION II. The formula mass (MM) of acetic acid is 60.052 g; the formula
mass of sodium bicarbonate is 84.007 g and the formula mass of sodium acetate is
82.034 g. Remember the definition of a mole you have worked with; i.e. moles (n)
can be calculated by dividing the mass (g) of a sample or a pure substance by its
molar mass (FM) in grams. This relationship is summarized by Equation 1.
EQ1: n = g/FM
You will need the data in Observation II to process the items below.
4. Calculate the mass of 0.0125 n sodium bicarbonate: ___________________
5. Calculate the theoretical yield (mass) of sodium acetate that would be produced
from 0.0125 n sodium bicarbonate: ________________________
Managers should contact the instructor to validate the group’s work to this point.
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OBSERVATION III:
A. General Procedure: The amount of sodium bicarbonate to be used is 0.0125
moles and this should be weighed to the nearest milligram within a tolerance
of plus or minus 6 milligrams. The acetic acid is added in the form of vinegar.
The amount of vinegar used for each sample should be 25 mL. Use a 125 mL
Erlenmeyer flask as a reaction vessel. About 20-25 mL of distilled water should
be added to the flask. The NaHCO3 should then be added to the flask and
mixed until all is dissolved. The vinegar should be added in small amounts to
prevent the solution from bubbling over. You should allow the bubbles to
subside and nearly stop before adding the next batch of vinegar. Boiling chips
should be added to the chamber and the mixture brought to a boil using a hot
plate. The mixture is heated until all the liquid has evaporated but not long
enough to oxidize the product which is indicated first by a deep yellowing and
then black color of the solid. The flask should be cooled to room temperature
before weighing.
6. How many grams of NaHCO3 are there in 0.0125 n of the substance?
_____________
7. Ask the instructor to demonstrate the procedure known as quantitative transfer
technique. Describe the demonstrated procedure here:
8. Why is it suggested that you add the NaHCO3 to water instead of the other way
around?
9. Why is it important not to burn the sodium acetate?
10. What do you think would be the largest cause of error in this procedure?
Discuss your responses to Items 6-10 with the instructor and then read through the
detailed procedure below before you attempt to execute it.
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Table 2. Data from the Stoichiometry of NaHCO3 and C2H4O2 Reaction.
Step Number and
TRIAL
TRIAL
TRIAL
AVERAGE/
Data Generated
1
2
3
OTHER
1. Mass of NaHCO3
2. Mass of Empty 125 mL flask
3. Mass of boiling
chips
4. Mass chips + flask1
5. Mass chips + flask + product
6. Mass of product recovered1
7. Percent precision1
8. Theoretical yield1,2
9. Percent Yield1,3
1Do
these calculations while there is no other experimental work to do.
is a standard stoichiometric calculation that starts with the actual mass of the NaCO 3
measured (Row 1) which is converted to moles NaCO3 which is converted to moles sodium acetate
which is converted to grams sodium acetate. This needs to b e done for each sample and then the
average is calculated and placed in the last column.
3This is the same calculation as for accuracy.
2This
11. Execute the following EXPERIMENTAL PROCEEDURE after instructor approval to
begin: You should do three determinations total – at least two at a time. The
specific procedure for each determination is as follows:
a. Set up hot plate in the hood and turn it on medium-high.
b. Weigh a dry, clean 125 ml Erlenmeyer flask; record mass in the appropriate
column of Row 2 of Table 2; add 20 mL distilled water to the flask.
c. Weigh 0.0125 moles +/- 6 mg of NaHCO3; record the mass in the appropriate
column of Row 1 of Table 2.
d. Quantitatively, transfer the NaHCO3 to the 125 ml Erlenmeyer flask and mix
until it dissolves.
e. Weigh 4 boiling chips; record mass in the appropriate column of Row 3 of
Table 2. Add chips to the NaHCO3 solution.
f. Measure 25.0 mL of vinegar; slowly add small amounts (3-5 ml) of vinegar to
the flask so that the fizz produced gets no higher than half the height of the
flask. Swirl the contents flask gently after each addition of vinegar until no
more fizzing can be seen. Repeat until all the vinegar has been added.
g. Place the flask on the hot plate and allow the liquid to boil off. (NOTE: Two
flasks can be used on the same hot plate.) Keep an eye on this process: it
may be necessary to increase the heat at first to achieve a vigorous boiling and
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h.
i.
j.
k.
POGIL LAB 12 Page 5 of 6
then reduce the heat if the boiling becomes too vigorous. The flask needs to
be removed before significant burning of the sodium acetate occurs but left on
the hot plate until all liquid is gone (including condensation) and the flask is
completely dry. The solid will turn a light yellow just as all the liquid is
evaporated. Any darker color may introduce a slight error but water left on or
in the flask will introduce a larger error.
Remove the flask from the hot plate and allow to cool to a point where it is
comfortable to handle. You might just set it aside and start on the next sample
or do some calculations.
Weigh the flask; record mass in the appropriate column of Row 5 of Table 2.
Rinse the flask with tap water until all the salt has been dissolved. Then rinse
with distilled water. Invert flask over a paper towel and let drain dry.
If only two samples have been started, repeat Steps a-k for third sample; else,
when flask is dry, put away all lab materials.
EXERCISE END. Managers should collect the GRF and RRF, clip them together with
a paper clip, and place in the back of the left pocket of the folder. The folder should
be closed and left on the table.
REPORTER, NOTIFY INSTRUCTOR WHEN FINISHED. WAIT FOR INSTRUCTIONS.
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APPENDIX A
Each member of the group should write their own lab report. The report should
conform to the style sheet posted on the course website and appear similar to the
sample lab report on the website. The following considerations should be taken into
account when you write your report:
1. Title and Authors: Choose a title that clearly identifies the content of the
report. Be sure to list all members of the group as authors with your name first
to indicate the investigator doing the writing.
2. The introduction should include but not limited to the following in narrative
form (not necessarily in the order mentioned):
a. The chemical reaction being studied, as well as why the particular example
was chosen for study. (Reference)
b. A discussion of the stoichiometry of the reaction including balanced
equation including why and how the theoretical yield can be calculated.
(Reference other than POGIL 11).
c. A hypothesis for the experiment. Remember that a hypothesis is a statement
of outcome as well as how to measure the outcome.
3. Materials and Methods: This section must be in narrative form. Remember,
in materials and methods section to describe the exact method you used and
how each parameter was measured. For example, if you used a 50 mL
graduated cylinder to measure 5 mL of liquid, you should include that in your
methods. If you followed the procedure as mentioned in PL11 then you can
reference it by a simple statement similar to this one: “The procedure used in
these experiments were as described in POGIL 11. (Reference).”
4. Results: The data relative to the support of the hypothesis should be place in a
table similar to Table 2. The table(s) should be labeled and titled with a phrase
that clearly describes the content of the table. Trivial data such as masses of
flasks or weighing boats should not be displayed but calculated values should
be included. The narrative that describes the results should refer to the table
directly in a manner similar to this statement: “The results in Table 3 supports
the assertion that…” The results section should never list things that are part of
the method like masses of flasks, chips, etc. Be sure to include in the data table
the precision, percent yield, and accuracy data.
5. The Conclusion: This section should relate the results to the hypothesis,
discuss the precision, percent yield, and accuracy data, and if needed sources of
error and how these errors might be avoided by future experimenters.