Part 1
S’MORES AND LIMITING REACTANTS
Name
Key
Two atoms or molecules must come together in just the right way in order for them to react.
As a result, it is virtually impossible to obtain 100% yield in a chemical reaction by combining the
reactants in exact proportions (unless you can wait an infinite amount of time). In order to
increase the odds that at least one reactant will react completely, we often add more than is
needed of another reactant. This reactant is said to be in excess. The reactant that is used up
in the reaction is called the limiting reactant because it limits the amount of product formed.
In part one of this activity, you will use a recipe for S’mores as an analogy for a chemical
equation in which reactants and products are in set proportions to each other. You will be given
varying amounts of each reactant. One of these reactants will limit the number of S’mores you
can produce. The other reactants will be in excess. After working with this culinary “reaction,”
you will identify the limiting and excess reactants in chemical reactions and perform
stoichiometric calculations based on the amount of the limiting reactant.
OBJECTIVES
 Investigate the concept of limiting reagents by constructing S’mores.
PROCEDURE
Obtain a plastic bag of S’mores ingredients, a sheet of drawing paper, and markers or map pencils.
Use the following recipe to construct as many S’mores as possible using your ingredients.
Recipe for 1 S’more
2 teddy grahams
1 chocolate chip
1 marshmallow
Answer are given for a sample of 10
teddy grahams, 6 chocolate chips, and
7 marshmallows
ANALYSIS & CONCLUSIONS
Complete the analysis questions (1-2) on a separate piece of paper.
1. On the sheet of drawing paper, draw a diagram that shows the following:
 number of each reactant
 number of complete S’mores
 number of left over reactants (reactants in excess)
 identity of the limiting reactant
2. Label your drawings (4 graham cracker halves, 2 marshmallows, 1 S’more, etc.). Use markers
and/or colored pencils to make your drawings attractive. Be creative!! Your paper should look
something like the following (with the addition of labels).
Names ______________
reactants

Period ____
products
Limiting reactant:
3.
Write a “balanced” equation to represent this “reaction”. Check it with the instructor!
_____2 G + 1 M + 1 C  G2MC___(G= teddy graham, M = marshmallow, C = chocolate )
4. This analogy to a chemical equation would represent what type of chemical reaction (Single
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Displacement, Double Displacement, Decomposition, Synthesis, or Combustion)? Circle One.
5. What “reactant(s)” are in excess? ____Marshmallows and Chocolate chips___________________
6. What “reactant(s)” limit the number of smores that can be made? _Teddy grahams______
7. Determine the maximum number of s’mores you could create from these ingredients._5_
EXTENSION
1. Obtain three of each s’more reactants
2. Determine the average mass of each reactant that makes up a s’more.
Total mass of three teddy graham crackers? Answers will vary.
Average mass of one teddy graham cracker square? _________
Total mass of three chocolate chips? Answers will vary.
Average mass of one chocolate chip? ___________
Total mass of three marshmallows? Answers will vary.
Average mass of a marshmallow? ___________
Given the following information determine the maximum number of smores that could be created
using your “balanced” smore equation. Show all your work.
45.0 grams of graham cracker Answers will vary. Sample of problem set-up:
45.0 gram teddy graham crackers x 1 teddy graham cracker
Ave. mass of 1 teddy graham
75.0 grams of chocolate
x 1 s’more
2 teddy grahams
32.0 grams of marshmallow
ANALYSIS & CONCLUSIONS
1. Explain how finding average mass of each “s’more reactant” is similar to using average atomic mass
to find the moles of a substance.
To find the mass of the s’more reactant, the mass of the reactant in grams is multiplied by 1 of the
s’more reactants over the average mass in grams (calculated in Extensions #2)of that reactant.
2. Explain how using the number of pieces of each s’more reactant to determine how many s’mores
you can make is similar to using a balanced chemical reaction to convert between moles of one
substance and moles of another substance.
The number of s’mores pieces is like the mole ratio in the equation, telling how many of each part or
reactant is needed to make a “whole” product.
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3. Define: Limiting reactant (reagent).
The limiting reactant is the reactant that is used up first in the reaction and limits the amount of product
that can be produced in the reaction.
WASH YOUR HANDS! Construct your own s’more up at the instructors table, using the appropriate
balance equation. Use a skewer to hold your marshmallow(s) and roast over a burner flame. Slide
the marshmallow off the skewer between two crackers and the layer of chocolate. Wait a few
seconds for the hot marshmallow to melt the chocolate and then enjoy eating your treat.
(This last part should only be done if you use graham crackers, chocolate squares, and big
marshmallows. The teddy grahams, chocolate chips, and small marshmallows are a more cost effective
alternative. We called them “traveling s’mores”.)
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Limiting Reactant Wet-Lab
Name___KEY_______
In this experiment, the result of adding different proportions of reactants will be studied for the reaction
of calcium chloride and sodium phosphate. The reaction conditions will be studied when the reactants,
calcium chloride and sodium phosphate, are in the exact mole ratios given in the balanced equation,
when calcium chloride is in excess and when sodium phosphate is in excess. Analysis of results will be
based on the height of calcium phosphate formed in each reaction. The theoretical yield produced in
each combination of reactants will also be calculated. The supernatant will be tested to see which
reactant is in excess.
Pre-lab questions:
Define the following terms:
LIMITING REACTANT – the reactant that is used up first and limits the amount of
product that can be produced in the reaction
EXCESS REACTANT – the reactant that is left over after the reaction has gone to
completion
Write a balanced equation for the reaction of calcium chloride and sodium phosphate. Using solubility
rules, determine which product is the precipitate.
3 CaCl2 (aq) + 2 Na3PO4 (aq)  6 NaCl (aq) + Ca3(PO4)2 (s)
* Ca3(PO4)2 will precipitate.
Equipment:
2 safety goggles
2 250-mL beakers
9 test tubes
1 test tube rack
2 graduated cylinder
Rubber stopper
1 spot plate
3 disposable dropper pipets
1 metric ruler
Materials:
0.1-M Calcium chloride (CaCl2·2 H2O)
0.1-M sodium phosphate (Na3PO4·10 H2O)
Safety Procedures:
1. Wear safety goggles at all times during the experiment.
2. All reactants and products can be safely disposed down the drain.
3. Wash hands thoroughly after you have finished the lab.
Procedure Day 1:
1. Label 9 test tubes 1 – 9 . Make sure these test tubes are clean and dry.
2. Label one 250 mL beaker calcium chloride and measure out 50 mL of 0.1-M calcium chloride
from the reactant table.
3. Label a second 250 mL beaker sodium phosphate and measure out 50 mL of
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0.1-M sodium phosphate from the reactant table.
4. Add the 0.1-M calcium chloride solution and 0.1-M sodium phosphate solution in the following
proportions for each test tube as indicated:
Tube
mL of
0.1-M
CaCl2
1
2
3
4
5
6
7
8
9
1 mL
2 mL
3 mL
4 mL
5 mL
6 mL
7 mL
8 mL
9 mL
Water
4 mL
4 mL
4 mL
4 mL
4 mL
4 mL
4 mL
4 mL
4 mL
mL of
0.1-M
Na3PO4
9 mL
8 mL
7 mL
6 mL
5 mL
4 mL
3 mL
2 mL
1 mL
Use two separate graduated cylinders and dropper pipets to measure and distribute the indicated
volumes. Do not dispose of the left over 0.1-M of calcium chloride and 0.1-M sodium phosphate. It
will be used again later in the lab.
5. Mix the solutions in test tubes 1 – 9, by using the rubber stopper to seal the test tube and invert
the test tube 3 times. Mix each test tube in the same way. Do not shake the test tubes
vigorously.
6. Let the test tubes stand in the test tube rack overnight.
Procedure Day 2:
1. Measure the height of precipitate formed in each test tube and record the data in the Data Table
1.
Data Table 1
Tube
1
Height
in cm
2
3
4
5
6
7
8
9
Answers will vary.
2. Test the supernatant in each of the nine test tubes by placing 1 or 2 drops of the supernatant from
each test tube in a well of the spot plate. Add 1 or 2 drops of calcium chloride left over from
Part 1 and record whether or not a precipitate forms in Data Table 2.
Data Table 2
Tube
1
Precipitate
Forms
Y
with
CaCl2?
2
3
4
5
6
7
8
9
Y
Y
Y
Y
N
N
N
N
3. Repeat the procedure in step 2, but add 1 or 2 drops of the sodium phosphate left over from Part
1 and record whether or not a precipitate forms in Data Table 3.
Data Table 3
Tube
1
2
3
4
5
6
7
8
9
Precipitate
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Forms
with
Na3PO4
N
N
N
N
N
N
Y
Y
Y
Analysis.
1. Referring back to Data Table 1, which test tube had the most precipitate? What is the ratio of the
reactants (in mL) in this tube?
Tube #6
6 mL CaCl2
4 mL Na3PO4
3 mL CaCl2 : 2 mL Na3PO4
2. Does the ratio of the reactants in question #1 match the ratio of the reactants in the balanced
chemical equation? Explain any differences or similarities.
Yes. The ratio matches the balanced equation. This ratio will yield the maximum
of product.
amount
3. Using Data Table 2 and Data Table 3, give the reactant in excess (CaCl2 or Na3PO4) in each test
tube. If neither reactant is in excess, report this as neither.
Data Table 2
Tube
1
2
3
4
5
Reactant
in
Na3PO4 Na3PO4 Na3PO4 Na3PO4 Na3PO4
Excess
6
7
8
9
Neither
CaCl2
CaCl2
CaCl2
4. For which test tube would you expect there to be neither reactant in excess? Why?
Tube 6 – The reactants are in the correct stoichiometric ratio, so neither should be
left
over.
5. Did you have a test tube that did not have either reactant in excess? If no, why?
Yes – Test tube #6. The reactants were mixed in the correct stoichiometric ratio so there is no
limiting or excess reactant.
If no,( it is possible ) because we used a graduated cylinder and the measuring
may not have
been precise, that there may not be a test tube where there is no
excess.
Extensions.
1. Calculate the moles of calcium chloride and moles of sodium phosphate in each test tube.
Tube
1
CaCl2
Tube
2
Tube
3
Tube
4
Tube
5
Tube
6
Tube
7
Tube
8
Tube
9
0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 0.0008 0.0009
Na3PO4 0.0009 0.0008 0.0007 0.0006 0.0005 0.0004 0.0003 0.0002 0.0001
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2. Calculate the maximum theoretical yield of precipitate that could form in each test tube.
Tube 1
Tube 2
Tube 3
0.00003 0.000067 0.0001
Tube 4
Tube 5
Tube 6
0.00013 0.00017 0.0002
Tube 7
Tube 8
0.00015 0.0001
Tube 9
0.00005
3. Which test tube would have the most precipitate formed? What is the mole ratio of the reactants
in this test tube?
Tube 6
0.0006 mol CaCl2
or
3 mol CaCl2
0.0004 mol Na3PO4
2 mol Na3PO4
4. How does the mole ratio of the reactants in #3 compare to the balanced chemical equation?
The mole ratio is the same in #3 as in the equation.
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Stoichiometry and Excel: Limiting Reagents
Excel Spreasheet: Hareesh Gadde and Jacob Stelle
Lesson: Hareesh Gadde and An-Phong Le
Goal: At the end of this exercise the student will understand the concept of a limiting reagent at the
macroscopic level (the amount of a reagent needed in grams or moles).
This Excel spreadsheet provides students the opportunity to visualize mole-mole calculations in
stoichiometry without the need to for students to perform the actual calculations. This may be helpful in
establishing a firm student understanding in the meaning of the stoichiometric coefficients in a balanced
reaction. The idea of a limiting reagent comes naturally as the students manipulate the percent
completion, and this spreadsheet may help clear students’ confusion regarding the use of moles versus
mass in stoichiometry.
Pre-activity Exercises
Reactants in a chemical equation combine to form products in set proportions. These proportions are
derived from the coefficients in the balanced chemical equation. This is not always a 1:1 ratio.
Write a balanced reaction using Na3PO4 and CaCl2 as the reactants:
3 CaCl2 + 2 Na3PO4  6 NaCl + Ca3(PO4)2
Given the amount of reagents in a-d below, figure out the following for each: 1) the amount of products
that will be made, 2) the amount of reagents left over, 3) how much more of either reagent you would
need in order to use the leftover.
a) 1 mole of Na3PO4 and 1 mole of CaCl2
The CaCl2 is the limiting reactant. (1) 0.33 mol Ca3(PO4)2 and 2.0 mol NaCl will form. (2) 0.34
mol of Na3PO4 will be left over. (3) 0.51 mol of CaCl2 would be required to use up the leftover
Na3PO4.
b) 2 moles of Na3PO4 and 1 mole of CaCl2
The CaCl2 is the limiting reactant. (1) 0.33 mol Ca3(PO4)2 and 2.0 mol NaCl will form. (2) 1.34
mol of Na3PO4 will be left over. (3) 2.01 mol of CaCl2 would be required to use up the leftover
Na3PO4.
c) 1 mole of Na3PO4 and 3 moles of CaCl2
The Na3PO4 is the limiting reactant. (1) 0.5 mol Ca3(PO4)2 and 3 mol NaCl will form. (2) 1.5
mol of CaCl2 will be left over. (3) 1 mol of Na3PO4 would be required to use up the leftover
CaCl2.
100 moles of Na3PO4 and 150 moles of CaCl2
d)
The reactants are in perfect stoichiometric ratio. (1) 50 mol Ca3(PO4)2 and 300 mol of NaCl
would form. (2) and (3) There are no reagents left over.
Excel Exercise
1) Open calciumphosphate-stoichiometry.xls from ______________________.
2) Make sure you are at the Reactant tab in the bottom left hand corner.
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3) Make sure the equation at the top of the page is balanced correctly. (NOTE: Reactants are listed
on the left of the arrow in 2 lines and products are to the right of the arrow in 2 lines.)
4)
5)
6)
7)
8)
Find the slider bar (
) in the middle of the document.
Slide the bar to the left so that there is a 0 (zero) under the % completion.
Click on the right arrow on the slider until the slider reads 25 for 25% completion.
Find the graph for Changes in Moles for each compound (bottom left).
Move your mouse over the bar graph to find the VALUE for each reactant and product. Record
these values in Table 1.
9) Complete the table for 50%, 75%, and 100% by adjusting the slider bar and reading the value for
each reactant or product.
TABLE 1
Percent
completion
CaCl2
Na3PO4
Ca3(PO4)2
NaCl
25%
- 0.742
- 0.495
0.247
1.484
50%
-1.484
-0.989
0.495
2.968
75%
-2.226
-1.484
0.742
4.452
100%
-2.968
-1.979
0.989
5.936
Answer the following questions using your table.
1. Why are there positive and negative numbers? What does a positive number represent? A
negative number?
Reactants are decreasing so they are negative and products are increasing so they are positive.
2. Does it make sense that the reactants are negative and the products are positive? Why?
Yes negatives are decreasing (and becoming less) and positives are
increasing(and
becoming more).
3. What do you notice about the amount of Ca3(PO4)2 and NaCl? How does this relate to the
balanced equation?
They increase as the reaction progresses. At 100%, the amount of each is the same as the
number of moles from the balanced equation.
4. What do you notice about the CaCl2 and Na3PO4? How does this relate to the balanced
equation?
They decrease as the reaction progresses. At 100%, the amount of each is the same as the
number of moles from the balanced equation.
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Understanding Reaction progress
1) Return to the excel document calciumphosphate-stoichiometry.xls.
2) Click on the blue tab (Starting Conditions) in the bottom left hand corner
3) Set up the reaction to start with 100 grams of Calcium Chloride and 100 grams of Sodium Phosphate
by adjusting the slider under the initial mass of each reactant.
4) Click on the green reaction tab in the bottom left hand corner.
3) At 0%, 25%,50%,75%, and100% completion, record the masses and change in number of moles in
the compound in Table 2.
TABLE 2-Change in number of moles
Percent
CaCl2
Na3PO4
completion
Ca3(PO4)2
NaCl
25%
-0.228
-0.152
0.076
0.455
50%
-0.455
-0.303
0.152
0.91
75%
-0.683
-0.455
0.228
1.365
100%
-0.91
-0.607
0.303
1.82
4) Compute the ratio of the changes in moles of Calcium Chloride:Sodium Phosphate, Calcium
Chloride:Calcium Phosphate, Sodium Phosphate:Calcium Phosphate.
Calcium chloride: Sodium phosphate
-0.228 / -0.152 or 3:2
Calcium chloride: Calcium phosphate - 0.228 / 0.076
or
3:1
Sodium phosphate: calcium phosphate 0.152 / 0.076
or
2:1
5)Return to the calciumphosphate-stoichiometry.xls document and examine the moles of reactants and
products graph in the upper left hand corner. At 100% completion what do you notice? What is the
significance of this observation?
At 100% completion, all of the calcium chloride has been used up (making it the limiting reactant).
There is an excess of sodium phosphate. The amount of product formed is in the ratio of 1 Ca3(PO4)2:6
NaCl.
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©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright
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