Limiting Reagent LABORATORY task.

Limiting Reagent
Department of Chemistry
This laboratory experiment has a formal report. Consult the guidelines for writing a
formal laboratory report prior to doing this laboratory experiment so that you know what
will be expected of you after the experiment is completed. The formal laboratory reports
generally take much longer to type than regular laboratory reports take to write. Post-lab
questions should be attached to your report.
Introduction
The concept of the limiting reagent can be demonstrated by an analogy: Jerry works in the
Purchasing Department at a specialty car manufacturing plant in Detroit. The plant gets an order for
100 limited edition cars for Macy’s parade. Jerry purchases 100 car bodies, 100 steering wheels, and
100 tires. Sadly, Jerry gets fired. Do you know why?
You probably realized that Jerry did not order enough tires to make the 100 cars required. With his
order Jerry could only make 25 cars, and he would have 75 leftover car bodies and steering wheels.
In this story the tires are the “limiting reagent” and the bodies and steering wheels are “in excess.”
Now let’s apply this concept to chemistry. Consider the balanced combustion reaction of methane
gas:
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)
According to the reaction one mole of methane gas will react with two moles of oxygen gas to
produce one mole of carbon dioxide gas and two moles of steam.
Worked example: When a reaction involves a solution, the concentration of the solution is
generally reported as molarity (shown in units of molar, M, which is equal to moles of solute per liter
of solution)—e.g. a 1.50M sodium chloride solution has 1.50 moles of NaCl per liter of solution or
L
1.50 mol NaCl . Given the volume and molarity of any solution, we can calculate the number of
moles of solute present. For example, given 50.0 mL of a 1.50M sodium chloride solution, we can
calculate the number of moles of NaCl present as follows:
50.0 mL = !"
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1.50 mol NaCl
1000 mL
1 L
0.0750 mol NaCl
What mass of precipitate could be produced if 10.0 mL of 1.500M aluminum chloride reacts with
10.0 mL of 1.500M sodium hydroxide? First, we write the balanced chemical equation:
AlCl3(aq) + 3 NaOH(aq) → Al(OH)3(s) + 3 NaCl(aq)
The precipitate formed is aluminum hydroxide. In this example we must first calculate moles for each
reactant, then continue with the molar ratio step, then convert moles of the precipitate back to grams.
Limiting Reagent
Glendale Community College Department of Chemistry. Version: F2014 2
10.0 mL !"
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1.500 mol AlCl
1000 mL
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1 mol AlCl
1 mol Al(OH)
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1 mol Al(OH)
78.0 g Al(OH)
= 1.17 g Al(OH)3
10.0 mL !"
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1.500 mol NaOH
1000 mL
1 L
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3 mol NaOH
1 mol Al(OH)3
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1 mol Al(OH)
78.0 g Al(OH)
= 0.390 g Al(OH)3
Notice that the two product amounts are very different. Even though there is enough
aluminum chloride to make 1.17 grams of Al(OH)3 precipitate, there is only enough
sodium hydroxide to make 0.390 grams of Al(OH)3. Once that amount of precipitate is
made, we run out of sodium hydroxide. Thus, in this example, sodium hydroxide is the
limiting reagent, aluminum chloride is in excess, and 0.390 grams of aluminum
hydroxide precipitate can be produced.
While these calculations may not be as exciting as manufacturing specialty cars, if a
chemist miscalculates limiting reagent problems he or she could get fired, cause a serious
accident, and (in the extreme case) kill someone due to an explosion or other serious
accident!
In this lab you will conduct several trials of a reaction, determine the limiting reagent and
the reactant in excess for each trial, and record your observations.
Limiting Reagent
Glendale Community College Department of Chemistry. Version: F2014 3
Procedure:
In your Lab Notebook, before beginning the experiment, prepare a data table to record the following
data for all six trials (A1, A2, A3, B1, B2 and B3) for each solution, CaCl2 (aq) and Na2CO3 (aq) :
initial volume
final volume
total volume
Directly below the data table, label a space to record the exact concentrations of CaCl2 (aq) and Na2CO3 (aq)
Part I. Precipitation reactions.
1. Clean and label six test tubes: A1, A2, A3, B1, B2 and B3. In each of the six test tubes, add
approximately 4 mL of calcium chloride solution to the six test tubes. Record initial and final
volumes for calcium chloride in the Lab Notebook. Also record the exact molarity from the
reagent bottle in the Lab Notebook. Calculate total volume delivered for each test tube and
record this result in the lab notebook.
2. Now, in test tubes A1 and B1, add approximately 1mL of sodium carbonate solution to each.
In each of test tubes A2 and B2, add approximately 4 mL sodium carbonate solution. In each
of test tubes A3 and B3, add approximately 6 mL of sodium carbonate solution. Record initial
and final volumes for sodium carbonate in the Lab Notebook. Also record the exact molarity
from the reagent bottle in the Lab Notebook. Calculate total volume delivered for each test
tube and record this result in the lab notebook.
3. Place each test tube in the Vortex Mixer (set to Auto/Touch; adjust the dial to a low-medium
setting (a 4-5 setting on the vortex) for about 25-30 seconds. Once all six solutions have been
thoroughly mixed, place them in two separate test tube racks, separated by letter (A1, A2 and
A3 in one rack and B1, B2 and B3 in a different rack). These two sets will be treated
differently. Place the “B” rack in an area on your bench-top where it will not be disturbed.
Record the time in your Lab Notebook and do not touch these test tubes (“set B”) until you are
ready for Part IV (after at least an hour has passed) so any precipitates can settle well.
Part II. Filtration.
1. Obtain three medium-flow filter papers and label them in pencil on their edges with a small
“A1”, “A2” and “A3”. Weigh each filter paper and record their masses in your Lab
Notebook. Obtain three 50 mL beakers and label them “A1”, “A2” and “A3”.
2. Set up a filtration apparatus with a plastic funnel in a ring stand. Fold and insert the filter paper
“A1” into the funnel and place the 50mL beaker “A1” beneath the funnel to catch the filtrate.
3. Pour the contents of test tube A1 into the funnel, using a D.I. wash bottle to wash any
remaining precipitate out of the test tube into the filter paper. When the liquid had passed
through filter, wash it with about 3mL of D.I. water. When this liquid had passed through the
filter, repeated the wash with another 3mL of D.I. water. SAVE the filtrate for each trial to use
in Part III.
Limiting Reagent
Glendale Community College Department of Chemistry. Version: F2014 4
4. Remove the filter paper and carefully open it up, placing it on a watch glass. Place the watch
glass on a hot plate (set to heat setting 4, or 70°C) to dry. Record the time drying started and
ended in your notebook under observations. While the filter paper “A1” is drying, repeat steps
2-4 above with “A2”. While the filter paper “A2” is drying, repeat steps 2-4 with “A3”. (You
may filter all three simultaneously with three ring clamps, three funnels, and three beakers if
you wish. Please dry each on a separate hot plate, though.)
5. Weigh each filter paper when dry and record its new mass in your Lab Notebook. Calculate
the actual yield of precipitate for each trial and record this in your Lab Notebook.
6. Write and balance the chemical equation for the reaction in your Lab Notebook. Identify the
precipitate in this reaction. Calculate the theoretical yield of each of these three trials, and then
determine the percent yield for each of these trials. Determine the limiting reagent and the
reagent in excess for each trial. Be sure to clearly label each trial’s calculation and results.
Results will be graded in your lab notebook!
Part III. Testing the Filtrates from Set A.
1. Test the ions dissolved in the filtrate solutions by transferring a small amount of the
solution from filtrate trial A1 to two different clean test tubes with a disposable pipette. Use
new disposable plastic pipettes for each test to prevent cross-contamination.
2. Add about 1 mL more CaCl2 (aq) solution to the first new test tube and about 1 mL more
Na2CO3 (aq) to the second. Record your observations in your notebook in Data & Results
Summary Table – Part III (see page 5 as a reference).
3. Repeat steps 1-2 for the filtrates from A2 and A3. Remember: use new pipettes for each test.
Part IV. Observing the Precipitates in Set B.
1. Now, without disturbing the precipitates, examine the test tubes in test tube rack B that were
set aside for at least an hour. Draw the three test tubes in your Lab Notebook, making sure to
note the level of the precipitates and the level of the liquid surface in each. Observe and record
the physical properties of the precipitates.
Finally, complete the Data Page and Postlab Questions, pages 6 -7. Use the Format for Formal
Lab Report on the CHM151LL website to help you write your Formal Report. Combine your
work in the following order and staple together to submit the following week: Formal Lab Report,
copies of your Lab Notebook pages, and the post-lab question sheets.
Limiting Reagent
Glendale Community College Department of Chemistry. Version: F2014 5
Please copy the following Data and Results Summary Tables in your lab
notebook.
Your data should be neatly entered into your lab notebook as the experiment
is carried out, not on this page.
Example Data & Results Summary Table – Parts I & II.
Trial A1 A2 A3 B1 B2 B3
Volume of
CaCl2 (aq)
Volume of
Na2CO3 (aq)
Theoretical
mass
of precipitate
Limiting
reagent
Reagent in
excess
Actual mass
of precipitate
Percent yield
Example Data & Results Summary Table – Part III.
Trial A1 A2 A3
Predicted reagent
in excess
Observation after
adding CaCl2 (aq)
Observation after
adding Na2CO3 (aq)
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Glendale Community College Department of Chemistry. Version: F2014 6
Postlab Questions.
Name: ________________________ Partners: ______________ _________
Section Number: ________________
Turn in pages 6-7 along with the formal report and pages from your lab notebook
1. Betty Smith is making little stick figure donkeys for her daughter’s birthday party. Each
donkey needs one head, one body, two ears, and four legs.
Draw a stick figure donkey here à
How many donkeys can she make with 18 heads, 20 bodies, 39 ears and 74 legs?
The limiting body part is
The body parts in excess are
How many of each excess piece will be left over?
2. Consider the reaction, N2 (g) + 3H2 (g) à 2NH3 (g) .
The picture to the right represents a mixture of N2 (g) and H2 (g) just before
reaction occurs. (Smaller lighter spheres are hydrogen molecules, and larger
darker spheres are nitrogen molecules.)
What is the limiting reactant?
How much of the excess reactant remains after the reaction is complete?
3. Consider the following reaction:
V2O5 (s) + 2NaOH (l) Δ ""→ 2NaVO3 (s) + H2O (g)
red colorless yellow colorless
solid liquid solid gas
When the red solid is mixed with the colorless liquid and heated, the mixture starts to
give off a gas and change color. When the reaction has stopped, the result is a dry, yellow
solid containing small, red particles. Which substance is the limiting reactant? Explain
how you determined this.
Limiting Reagent
Glendale Community College Department of Chemistry. Version: F2014 7
4. In the beakers below, using your calculation results, draw what the filtrate solutions
contained for each trial. Show the precipitate and the excess reagents in the beakers.
(Recall how we showed solution contents in the pictures from the Ionic Solubility Lab.)
Trial A1 Trial A2 Trial A3
5. If calcium chloride is the limiting reagent and sodium carbonate is in excess:
a. Which ions are present in the solution above the precipitate? _______________
b. What would happen if you added CaCl2 (aq) to the solution?
c. What would happen if you added Na2CO3 (aq) to the solution?
6. If sodium carbonate is the limiting reagent and calcium chloride is in excess:
a. Which ions are present in the solution above the precipitate? _______________
b. What would happen if you added CaCl2 (aq) to the solution?
c. What would happen if you added Na2CO3 (aq) to the solution?
7. In the space below, neatly draw three identical, empty test tubes. Label the test tubes
“B1”, “B2” and “B3”. In the three test tubes, draw a horizontal line to show the level
of the liquid and another horizontal line to show the amount of precipitate produced in
lab as the volume of sodium carbonate increased with each trial.