Experiment 6 – Percent Composition
Objectives
1. To observe the techniques involved to carry out the decomposition of a
chemical, potassium chlorate, KClO3, quantitatively.
2. To calculate the percentage of oxygen in potassium chlorate, KClO3,
theoretically and experimentally.
3. To verify the product of the decomposition reaction is potassium chloride,
KCl.
Part A Instructor Demonstration
Apparatus:
1.
2.
3.
4.
Porcelain crucible with lid
Tongs
Triangle
Ring clamp
Part B
Apparatus:
1.
2.
3.
4.
2 test tubes
Test tube rack
Stir rod
Water bottle
CHEM 0011 – May 2010
5.
6.
7.
8.
Fisher burner and lighter
Desicooler
Spatula
Analytical balance
Part C Instructor Demonstration
Apparatus:
1. Test tube
2. Test tube rack
Solutions:
1. Dilute nitric acid, HNO3, solution
2. 0.1 M silver nitrate, AgNO3, solution
Solids:
1. Solid potassium chlorate, KClO3
2. Solid potassium chloride, KCl
CHEM 0011 – May 2010
Introduction
The decomposition of potassium chlorate occurs at temperatures above 400oC.
The reaction is described by the word equation as:
potassium chlorate + heat
potassium chloride + oxygen
The balanced chemical equation is:
Your instructor will set up the apparatus to carry out the decomposition reaction in
the fume hood. Using proper weighing and heating techniques, your instructor will
demonstrate the procedure to decompose a small quantity of potassium chlorate
quantitatively to ensure that all the weighed potassium chlorate undergoes
heating and no solid sample is lost during the heating process.
A weighed sample of potassium chlorate will be heated in a porcelain crucible at
temperatures above 400oC until all of its oxygen content will be driven off as O2
gas. After the heating, the mass of the solid residue, which remains in the
crucible, will be determined. According to the chemical equation, we will observe
a decrease in mass after heating since the molar mass of KCl is less than the
molar mass of KClO3. The loss in mass will be the amount of oxygen that is
driven off as O2 gas. Therefore, using the following equation and experimental
data, the percentage of oxygen in the original sample of KClO3 can be
determined.
The percentage of oxygen in KClO3 can be determined theoretically using the
following equation.
CHEM 0011 – May 2010
In Part C, in order to confirm that the residue in the crucible is KCl, your instructor
will demonstrate by reacting the residue in the crucible with a silver nitrate,
AgNO3, solution that is acidified with nitric acid, HNO3. If the residue is potassium
chloride then the acidified solution will react with the silver nitrate solution to
produce silver chloride, AgCl. Silver chloride will appear as a white precipitate in
the test tube.
In Part B, you will perform the same test using KCl. When KCl is tested, silver
chloride, AgCl, will form as a precipitate. The chemical reaction is as follows:
When the same test is carried out with KClO3, the result will be not be the same.
From your observations in Part B and C, you will be able verify that the solid
product formed from the decomposition of KClO3 is KCl.
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Procedure:
Part A - Decomposition of Potassium Chlorate Reaction
Instructor Demonstration
1. Obtain a clean crucible and lid. Inspect the crucible for cracks.
2. Use an analytical balance and determine the mass of the empty crucible.
Record the mass of the empty crucible to four decimal places on the data
sheet.
3. Use an analytical balance and weigh approximately 2.4 to 2.6 grams of
solid potassium chlorate, KClO3, into the crucible. Record the mass of the
crucible and the KClO3 to four decimal places on the data sheet.
4. Place the crucible on the clay triangle and heat gently for about 8
minutes followed by strong heating for 10 minutes.
5. Allow the crucible to cool to room temperature inside the desicooler and
weigh the crucible and the residue. Record the mass of the crucible and
the residue (after 1st heating) to four decimal places on the data sheet.
6. After weighing, place the crucible back on the clay triangle and heat
strongly for about 5 minutes. Allow to cool and reweigh. Record the
mass of the crucible and the residue (after re-heating) to four decimal
places on the data sheet. If you have done a good job in heating, the mass
of the crucible and residue after the 1st heating and the 2nd heating should
not differ by more than 0.05 gram.
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Procedure:
Part B - Examination of Potassium Chlorate and Potassium Chloride
1. Place 2 test tubes in a test tube rack. Label the test tubes as #1, and #2.
2. Use a spatula to transfer ½ a spatula tip of the following samples into each
of the test tubes. Record your observations on the data sheet.
Test tube
Sample
#1
Potassium chloride, KCl
#2
Potassium chlorate, KClO3
3. To each test tube add about 5 mL (approximately ½ a test tube) of distilled
water to each test tube and shake to dissolve. Record your observations
on the data sheet.
4. To each test tube add about 3 drops of dilute nitric acid, HNO3, and mix.
Record your observations on the data sheet.
5. To each test tube add about 5 drops of AgNO3 and mix. Record your
observations on the data sheet.
NOTE: If silver chloride, AgCl, is present, it will appear as a white precipitate.
Part C – Examination of the Residue from the Decomposition of KClO3
Instructor Demonstration
1. Place 1 test tube in a test tube rack. Label the test tube as #3.
2. Use a spatula to transfer ½ a spatula tip of the residue from the crucible
into the test tube. Record your observations on the data sheet.
3. Add about 5 mL (approximately ½ a test tube) of distilled water to the test
tube and shake to dissolve. Record your observations on the data sheet.
4. Add about 3 drops of dilute nitric acid, HNO3, and mix. Record your
observations on the data sheet.
5. Add about 5 drops of AgNO3 and mix. Record your observations on the
data sheet.
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6. Compare the content of test tube #3 with test tubes #1 and #2.
Datasheet:
Part A: Decomposition of Potassium Chlorate Reaction
1. Mass of empty crucible
2. Mass of crucible and KClO3
3. Mass of crucible and residue
(after 1st heating)
4. Mass of crucible and residue
(after re-heating)
5. Mass of KClO3 used
6. Mass of residue which remains
7. Mass of oxygen released
CHEM 0011 – May 2010
Datasheet:
Part B and Part C: Examination of KCl, KClO3, and Residue
Record
observations of
the content in
the test tube
after each step:
Step 2:
Solid
(Colour and
appearance)
Step 3:
After addition of
5 mL of water to
the solid in the
test tube
Step 4:
After addition of
3 drops of HNO3
solution
to the test tube
Step 5:
After addition of
5 drops of
AgNO3 solution
in the test tube
CHEM 0011 – May 2010
Test tube #1
Potassium Chloride
(KCl)
Test tube #2
Potassium Chlorate
(KClO3)
Test tube #3
(Instructor
Demonstration)
Residue
Calculations:
Calculation of % oxygen (experimental):
Calculation of % oxygen (theoretical):
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Percent Error Calculation
Using the results from the two previous calculations, the % error for the experimentally determined
percent oxygen in KClO3 can be carried out by the following equation:
% Error = |theoretical % oxygen in KClO3 - experimental % oxygen in KClO3 | * 100
theoretical % oxygen in KClO3
Show work here and report your answer with proper number of significant figures.
CHEM 0011 – May 2010
Questions:
1. In Part A, what would be the effect of insufficient heating on your
experimental result. Would the experimentally determined percent oxygen
in KClO3 be higher or lower than the theoretical prediction?
2. In Part B and Part C, which two test tubes behaved similarly, and what can
be said about the residue as a result of the comparison of the three test
tubes.
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3. Write the balanced chemical equation and word equation for the chemical
reactions, which occurred in:
(a) Part A.
(b) Part C.
4. Calcium chlorate, Ca(ClO3)2, when heated, also releases oxygen. The
residue which remains is calcium chloride. Write the word equation for this
reaction.
CHEM 0011 – May 2010