Introduction
Many chemical reactions are conducted by combining
solutions of reagents that will react rather than by reacting the pure
materials. This strategy is particularly useful when the reactants are
pure solids. Also, when synthesizing a compound it is common to have an
excess of one of the reactants present in order to assure complete reaction
of the second reactant. In such a reaction, the reactant present in excess
is called the excess reactant, and the reactant that limits the extent
of the reaction is called the limiting reactant (see Stoichiometry module).
It is important to know which solution placed into the reaction vessel
is providing the limiting reactant in order to know the maximum amount
of product (theoretical yield) that can be obtained.
2. If you should come into contact with any reagents you should immediately wash the affected area with large amounts of water.
3.
Be sure to operate the centrifuge according to the procedure given by your
teacher. Be sure to place a second test-tube containing the same amount
of
material in a test-tube holder opposite the holder containing the sample.
Do not attempt to stop the centrifuge by making contact with the spinning
head.
If a test-tube should break while in the centrifuge, notify your teacher
at once—do not attempt to clean the centrifuge without direction from your
teacher and only after the centrifuge has been disconnected from the electrical
outlet.
4.
Be careful not to contact the hot surfaces of the oven when placing objects
into or removing objects from the oven. Also, objects being removed
from the oven are very hot and should be handled with tongs or other appropriate
utensils as demonstrated by your teacher.
5. Do not pour any substances from this experiment down the drain. Dispose of them as indicated by your teacher.
2. Place 10 mL 1.0 M BaCl 2 solution into
a 50-mL beaker and label the beaker. Place 15 mL 0.50 M Na 2
SO 4 solution into another
50-mL beaker and label the
beaker.
3. Take clean and dry test-tubes and label the test-tubes
1, 2, 3, and 4 using a pencil. Measure and record the mass of each of the
test-tubes to the nearest
0.01 g.
4. Use one of the BeralÔ pipets to measure
2.0 mL 1.0 M BaCl 2 solution from your beaker into each
of the four numbered test-tubes. Be careful when
filling the pipet to avoid
air bubbles in the liquid column in the pipet. Use one of the 250-mL beakers
as a test-tube rack. Make sure that the pipet
used with the BaCl 2
solution stays in the beaker containing the BaCl 2 solution after you have
finished using it.
5. Use a second BeralÔ pipet to measure volumes
of 0.50 M Na 2 SO 4 solution indicated below into
the test-tubes. Expel the Na 2 SO 4 solution into
the test-tubes in a fast
stream to promote mixing of the solutions and make sure that the tip of
the pipet does not go into the solutions in the test-tubes.
6. Place 2.0 mL 0.50 M Na 2 SO 4
into test-tube number 1, 3.0 mL into test-tube 2, 4.0 mL into test-tube
3, and 5.0 mL into test-tube 4. Make
sure that the BeralÔ
pipet used for the Na 2 SO 4 solution stays in the
beaker containing the Na 2 SO 4 solution after you
have finished using it.
7. Stir the solutions with the stirring rod; rinse
the stirring rod with a stream of distilled water from your wash bottle
each time it is withdrawn from a
test-tube making sure that
the washings go into the test-tube from which you are withdrawing the rod.
8. Using water from your wash bottle fill all the
test-tubes to the same height. The liquid should be about 2 cm from the
top of the test-tubes. Do not
overfill! Place the test-tubes
into the centrifuge using two of them to counterbalance the other two and
spin them for about one minute. Remove the
test-tubes from the centrifuge.
The solid BaSO 4 should be at the bottom of the test-tube, and
the supernatant liquid (the liquid above the precipitated
solid) should be clear,
or slightly cloudy. If the liquid is very cloudy, check with your teacher
to determine if you should continue to centrifuge the tubes.
Your teacher may also recommend
additional steps to aid the settling of the precipitate.
9. Use the third BeralÔ pipet to remove the
supernatant liquid from above the precipitates and discard the liquid into
the 250-mL beaker not being used as
a rack. Be careful not to
remove any of the settled precipitate from the test-tubes.
10. Fill each test-tube about 2/3 full with distilled
water from your wash bottle. Stir each solution with the stirring rod so
that the settled precipitate is
dispersed into the liquid.
Use the wash bottle to rinse the stirring rod as you did in Step 7. Now
add distilled water from your wash bottle to the
test-tubes to make the level
of liquid the same in each test-tube, but at least 2 cm from the top. Centrifuge
the tubes as you did in Step 8. Once again,
the solid should be settled
and the supernatant liquid clear. Remove the supernatant liquid from the
test-tubes with the third BeralÔ pipet taking care not
to remove any of the solids.
11. Place the test-tubes containing the wet solids
into a 250-mL beaker and place the beaker into the drying oven. Be sure
it has an ID mark, such as your
initials, on it.
12. When the solids have dried remove them from the
oven. CAUTION: The beaker is hot and should be handled with tongs or other
approved handling
equipment. Place the beaker
on a wire gauze on your desk and allow the beaker, the test-tubes and the
solids to cool. Measure and record the mass
of each of the test-tubes
to the nearest 0.01 g.
13. Dispose of the test-tube contents, the excess
reagents, and waste solutions as directed by your teacher. (Do not pour
the chemicals down the drain!)
Wash all glassware and the
pipets and return all items to the appropriate place.
14. Thoroughly wash your hands before leaving the
laboratory.
1.
Make a table with the following column headings (use the long dimension
of your paper for the top of the table): [1] Test-tube number, [2]
Mass tube (g), [3] mL 1.0 M BaCl 2 , [4] mL 0.50 M Na 2
SO 4 , [5] Mass tube + BaSO4 (g), [6] Mass BaSO 4
obtained (g), [7] Moles BaSO
4 obtained, [8] Moles BaCl 2 added, [9] Moles Na
2
SO 4 added, [10] Moles BaSO 4 expected if all BaCl
2
reacted, [11] Moles BaSO 4
expected if all Na 2 SO 4 reacted, and [12] Limiting
reactant.
2. Fill in columns [1] through [5] from information from your data sheet.
3. Calculate the mass and moles BaSO 4 obtained in each test-tube and record the values in columns [6] and [7], respectively.
4. Calculate the moles BaCl 2 and Na 2 SO 4 added to each test-tube and record the values in columns [8] and [9], respectively.
5.
Write a balanced equation for the reaction that occurred and use information
from columns [8] and [9] to calculate the moles BaSO 4 expected
if (a)
all the BaCl 2 added had reacted and (b) all the Na 2
SO 4 reacted for each of the test-tubes, and record the values
in columns [10] and [11],
respectively.
6.
Compare the values in columns [7], [10] and [11] and decide which of the
reactant solutions, BaCl 2 or Na 2 SO 4
, contained the limiting reactant and
indicate your choice in column [12].
2. Does the smaller volume of solution always contain the limiting reactant? How do the experimental results verify your conclusion?
3. What pieces of information do you need before calculating the maximum amount of product to be expected from a reaction where the reactants are in solution?
4. Corn sweetener is a sweetening agent obtained from corn. A recipe calls for 8.0 ounces of pure corn sweetener. You go to the supermarket and find that there are two brands of corn sweetener both in solution form. Brand A comes in a 16 ounce bottle, is 40% corn sweetener by mass and costs $ 0.59. Brand B comes in a 24-ounce bottle, is 45% corn sweetener by mass, and costs $ 0.95. Which brand costs less per ounce of pure corn sweetener? How many ounces of the less expensive brand should be added to the recipe in order to get the 8.0 ounces of pure corn sweetener?
5. Barium ion in solution is highly toxic if ingested. Why can large
quantities of BaSO4 be ingested by individuals undergoing gastrointestinal
X-rays without any apparent ill effects?
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