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Determining the Percent Composition
of Potassium Chlorate in a Mixture
PRE-LAB ASSIGNMENTS:
To be assigned by your lab instructor.
STUDENT LEARNING OUTCOMES:


Learn how to use mole-mole stoichiometry.
Learn how to use chemical properties to analyze the components of a mixture
EXPERIMENTAL GOALS:
The purpose of this experiment is to determine the percent composition of potassium chlorate in
a mixture of potassium chlorate and sodium chloride by decomposing the potassium chlorate
with heat into molecular oxygen. The mass of the oxygen can be used to calculate the mass of
potassium chlorate in the original sample by using the stoichiometry of the decomposition
reaction.
INTRODUCTION:
This lab will introduce the concept of reaction stoichiometry. We will use the mole ratios
in a balanced chemical equation to calculate the amount of potassium chlorate in a sample by
measuring the amount of molecular oxygen that is produced when it undergoes a decomposition
reaction.
In the previous experiment, a set of compounds were separated by exploiting differences
physical properties, which allowed us to figure out the percent composition of the original
sample. In this experiment, a difference in chemical properties will lead to a change which can
be measured in the lab, which will allow the composition of the original sample to be
determined.
You will receive an unknown sample in a test tube which contains a mixture of potassium
chlorate, KClO3, and sodium chloride, NaCl. When heated, KClO3 decomposes into potassium
chloride and molecular oxygen:
2KClO3(s)  2KCl(s) + 3O2(g)
The NaCl does not react, and is chemically unchanged. The sample weighs less after heating,
because of the oxygen gas which is evolved during the reaction, so by knowing the weight of the
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sample before and after heating, the mass of the oxygen gas can be determined. Once the mass
of O2 is known, the number of moles of O2 can then be determined.
Stoichiometry is the study of the numerical relationships in chemical formulas and
reactions. Knowing the stoichiometry of a formula allows us to relate moles and grams for
particular reactants or products (e.g., that 1 mole of H2O weighs 18.02 g). Knowing the
stoichiometry of a reaction allows us to relate amounts of different substances to each other, and
allows us to predict how much of the product(s) will be formed, or how much of the reactant(s)
will be needed.
Since moles combine in the same ratio that atoms, molecules, or formula units do, the
coefficients in a balanced chemical reaction specify the relative amounts in moles of each of the
substances involved in the reaction.
For example, consider the reaction between hydrogen and oxygen to make water:
2H2(g) + O2(g)  2H2O(g)
The coefficients of this reaction can be interpreted in two different ways:
react with
this many
molecules
of O2
This many
molecules
of H2
to make
this many
molecules
of H2O
2 H2(g) + 1 O2(g)  2 H2O(g)
This many
moles
of H2
react with
this many
moles
of O2
to make
this many
moles
of H2O
This means that the coefficients can be used as conversion factors to relate the number of moles
of one chemical to the number of moles of another chemical:
2 moles of H2 = 1 mole of O2
2 moles of H2 = 2 moles of H2O
1 mole of O2 = 2 moles of H2O
or, if we rewrite these relationships as ratios:
2 mol H 2
1 mol O 2
2 mol H 2
2 mol H 2 O
1 mol O 2
2 mol H 2 O
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(The reciprocals of these relations are of course also valid ratios.) These relationships are true
only for this particular balanced equation. A different chemical equation will have its own
unique set of relationships, derived from the coefficients in that reaction.
For example, suppose we have 32.0 grams of O2 in the above reaction. We can use the
balanced equation to determine how many grams of H2 will be needed, and how many grams of
H2O will be produced:
Convert g O2 to mol O2:
32.0 g O 2 
1 mol O 2
 1.00 mol O 2
32.00 g O 2
Convert mol O2 to mol H2:
coefficient of what we’re interested in

2 mol H 2
1.00 mol O 2 
 2.00 mol H 2
1 mol O 2

coefficient of what we’re canceling out
Convert mol H2 to g H2:
2.00 mol H 2 
2.02 g H 2
 4.04 g H 2
1 mol H 2
or, we can put everything together in a single calculation,
32.0 g O 2 
1 mol O 2
2 mol H 2 2.02 g H 2


 4.04 g H 2
32.00 g O 2 1 mol O 2 1 mol H 2
How many grams of water will be formed?
32.0 g O 2 
1 mol O 2
2 mol H 2 O 18.02 g H 2 O


 36.0 g H 2 O
32.00 g O 2 1 mol O 2
1 mol H 2 O
In this experiment, the number of moles of O2 can be used to calculated the number of
moles of KClO3 in the original sample, and therefore the number of grams of KClO3 in the
sample. From the mass of KClO3 and the mass of the sample, the percentage of KClO3 in the
sample can then be determined.
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(a)
(b)
Figure 1. (a) Crucible in clay triangle. (b) Close-up of crucible.
PROCEDURE:
1. Obtain a sample of an unknown mixture of KClO3 and NaCl from the stockroom, and record
the unknown number on your report sheet.
2. Clean a crucible and lid with deionized water, place them in a clay triangle as shown in
Figure 1, and heat the crucible and lid for about five minutes. Allow the apparatus to cool,
and record its weight (2).
3. Add about 2 g of the unknown sample to the crucible, and reweigh (1). (Handle the crucible
and lid with your crucible tongs.) Assemble the apparatus as shown in Figure 1, leaving the
crucible slightly uncovered to prevent splattering.
4. Heat the mixture gently for about five minutes, then vigorously for another five to ten
minutes. Cool the crucible and weigh it (4).
5. Reheat for about five minutes, cool, and reweigh. Continue reheating, cooling, and
reweighing until a constant weight is obtained on two successive weighings. Record the final
weight (5).
6. Clean the crucible, heat it to dryness, reweigh it, and repeat the experiment.
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CALCULATIONS:
1. Determine the mass of O2 produced in the reaction and the number of moles of O2. Use the
stoichiometry of the reaction to find the number of moles of KClO3 and the mass of KClO3 in
the sample.
2. Determine the mass of NaCl in the original sample by difference.
3. Determine the percentage of KClO3 in the sample:
mass % KClO 3 
grams KClO 3
 100
grams sample
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LAB REPORT
Determining the Percentage of Potassium Chlorate in a Mixture
Name ________________________________
Date _________
Partner ________________________________
Section _________
Unknown No. ________
Report Grade ______
First
Determination
Second
Determination
1. Mass of crucible, lid and mixture
__________
__________
2. Mass of crucible and lid
__________
__________
3. Mass of sample
__________
__________
4. First mass of crucible, lid, and residue
__________
__________
5. Final (constant) mass of crucible, lid, and residue
__________
__________
6. Mass of O2
__________
__________
7. Moles of O2 (show calculations)
__________
__________
8. Moles of KClO3 in sample (show calculations)
__________
__________
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First
Determination
Second
Determination
9. Mass of KClO3 in sample (show calculations)
__________
__________
10. Weight percent of KClO3 (show calculations)
__________
__________
11. Average mass percentage of KClO3
12. Mass of NaCl in sample (show calculations)
__________
__________
__________