Safety
Read the Safety Considerations in the Student Version. Barium ion in solution,
or in a soluble compound, is highly toxic if ingested; therefore,
students should wash their hands thoroughly before leaving the laboratory.
Barium compounds must not be discarded down the drain. There
should be a waste container for these salts and solutions. After all students
have completed the activity, you can convert the sample to very
insoluble barium sulfate by adding excess sodium sulfate. The barium sulfate
waste can be filtered, dried, and stored for proper disposal
Safety precautions related to using a centrifuge should be stressed—be
sure to place a second tube containing the same amount of material in
a tube holder opposite the holder containing the sample. Because students
may be in a hurry at the time when they are using the centrifuge,
stress that the centrifuge should not be manually stopped by the insertion
of fingers or other objects. If safety considerations don’t impress
students you may point out that quickly stopping a centrifuge tends to
bring the solid back up into the liquid, thereby undoing what centrifuging
has just accomplished. Also warn students to keep ties and long hair away
from the spinning centrifuge. Occasionally a test-tube breaks while
being centrifuged. This usually does not result in flying glass but students
should be given instructions about cleanup procedures. Probably the
most common accident involves finger burns when placing glassware into
or removing it from the drying oven or handling glassware that has
not sufficiently cooled. Proper techniques for handling hot glassware should
be reviewed before the experiment.
Materials (For 24 students working in pairs)
Balances, CentigramÔ or equivalent
Centrifuge
Drying oven
Nonconsumables
48 Test-tubes, 13 x 100-mm or larger to fit available centrifuge
24 Beakers, 50-mL
24 Beakers, 250-mL
36 Plastic BeralÔ pipets, stems calibrated to 1.0 mL
12 Stirring rods
12 Wash bottles filled with distilled water
Consumables
1.0 M Barium chloride, BaCl 2 ·2H 2 O,
200 mL (48.8 g BaCl 2 ·2H 2 O per 200 mL)
0.50 M Sodium sulfate, Na 2 SO 4 ·10H
2 O, 250 mL (40.3 g Na 2 SO 4 ·10H
2 O per 250 mL)
Advance Preparation
Solutions should be prepared as above. If the Na 2 SO 4
in your stockroom is in the anhydrous form, 17.8 g per 250 mL yields the
desired 0.50 M
solution. All glassware should be clean and dry. If students
have to clean and dry the glassware, particularly the test-tubes, before
beginning they will
not have sufficient time to complete the activity. If the plastic BeralÔ
pipets are to be reused they should also be carefully washed to avoid
cross-contamination during reuse and all visible traces of moisture
removed. The BeralÔ pipet stems may be calibrated for 1 mL
by filling a 10-mL
graduated cylinder to the 10 mL mark with distilled water and drawing
sufficient water into the pipet to bring the water in the cylinder to the
9 mL mark. The water level in the pipet stem should then be marked with
a waterproof felt marker. The drying oven should be turned on at
the beginning of the
class so that it comes to drying temperature (120-130 °C) by the
time it is to be used. The thermostat settings should be determined the
day before the
laboratory. Always try demonstrations before doing them for a
class. Wear safety goggles when doing any chemical demonstration. Concentrated
ammonia is an eye, nose, and throat irritant. Be sure you have adequate
ventilation. Dry ice is extremely cold and can cause burns. Handle with
care. Use gloves.
The pre-laboratory discussion should take place the day before the activity is to be performed if it is to be completed in one 80-min period.
1. Review safety considerations and disposal methods involved with the handling of the chemical substances being used.
2.
Emphasize the necessity not to mix the BeralÔ pipets during the activity-one
should be used for the barium chloride solution and another used for the
sodium sulfate solution, and the third for removal of the supernatant liquid.
3. Make sure that students understand the difference between amount and concentration of a solution.
4. Review mole and molarity calculations as well as reaction stoichiometry calculations. The net-ionic equation for the reaction in this experiment is:
Ba 2+ (aq) + SO 4 2– (aq) ® BaSO 4 (s)
The overall reaction is:
Therefore, one mole of BaCl 2 reacts with one mole of Na 2
SO 4 . You should be able to elicit this information from your
students if they have studied
chemical stoichiometry.
5. Review the procedure used and safety considerations in operating the centrifuge and the drying oven.
6.
Explain that students can use the calibration marks on the beakers to measure
the 10- and 15-mL volumes needed in Step 2 as only approximate
volumes are necessary.
7.
Because the test-tubes are to be put in the oven, labeling in Step 3 should
not be made with a wax marking crayon or with paper labels.
Teacher-Student Interaction
One
problem that arises is the fineness of the barium sulfate precipitate.
Even after extended centrifuging the precipitate may not settle; some precipitate
may adhere to
the sides of the test-tube. The adherence to the test-tube is not a problem,
since the precipitate will remain in the test-tube and be weighed.
The precipitate
that does not settle is a very small fraction of the total precipitate
and, if removed with the supernatant liquid, will cause a small percent
error.
However, you
may want to tell students to add a small quantity of soap (not detergent)
solution by dipping the stirrer in a soap solution, mixing the solution
with the stirrer,
and then recentrifuging. This will usually aid the settling of the
precipitate. Monitor student use of the centrifuge. Any large vibration
usually
signals that
the centrifuge is not balanced. When in a hurry, students tend to disregard
the admonition against using fingers or other objects to stop the
centrifuge.
Monitor the use of the drying oven. Students may need help in removing
the hot beaker and test-tubes. It is best if all students place their
beakers into
the oven within minutes of each other. This strategy permits the oven to
heat up to the required drying temperature and remain there, speeding
drying.
Circulate around the room while students are doing their weighings. It
is important that the weighings be done properly and masses recorded
to
0.01g.
Anticipated Student Results
* The actual yield should be close to the theoretical yield. The factor
10 4 means that the quantities in columns 7-11 have been multiplied by
that
amount to yield numbers in the tens range. The actual quantity is obtained
by reversing the process and dividing by 10 4 ; e.g., 8.6 = 8.6 x 10 –4
(in Column 7; see Introductory Module for use of exponents).
Answers
to Implications and Applications
1. No. Sodium sulfate was
the solution of lower concentration, but the results in Tube 4 show that
barium chloride was the limiting reactant.
2. No. There were 2.0 mL barium chloride solution and 3.0 mL sodium sulfate solution in Tube 2 and sodium sulfate was the limiting reactant.
3. You must know the stoichiometric
ratio from the balanced equation, the concentrations of the reactants and
the volumes of solutions mixed together
in order to determine the limiting reactant and consequently the maximum
amount of product to be expected.
4. Brand A costs $ 0.092
per ounce of corn sweetener. Brand B costs $ 0.088 per ounce of corn sweetener.
Brand B (17.8 ounces) has to be used to
obtain 8.0 ounces of corn sweetener.
5. Barium sulfate is so insoluble
that a negligible amount of barium ions go into solution. The barium sulfate
ingested in “liquid” during a gastrointestinal
X-ray is really a slurry of barium sulfate in water similar to the slurry
obtained in this activity when the barium sulfate precipitate is washed
with distilled water.
Possible Extensions
Depending upon the students’ ability, the topics covered prior to the Solutions
module and the time allotted,
the following topics might be discussed:
1. The overwhelming number of chemical processes that take place in solution.
2. Introduction to the use of stoichiometric ratios in quantitative analysis.
3. The need to introduce excessive concentrations of reactants to shift
equilibrium.
Assessing Laboratory
Learning
1. Laboratory Practical.
The use of the centrifuge may be tested as part of a laboratory practical.
Also, an “unknown”
concentration of one of the solutions in this experiment may be presented
and students asked how they would go about
determining the concentration of that solution—or asked to actually do
it.
2. Written Examination. Problems
involving the use of solutions and limiting reactant determination could
be presented. Students
could be asked to construct a “pictures in the mind” illustration of the
limiting reactant for the BaCl 2 -Na 2 SO 4
reaction
similar to the H 2 -O 2 reaction illustrated in the Stoichiometry module.
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