Experiment 3 – Percent Composition
Objective - Determine the percentage of oxygen in potassium chlorate (KClO
3
).
Introduction
The decomposition of potassium chlorate (at temperatures above 400 o C) is described by the equation:
A weighed sample of potassium chlorate will be heated until all the oxygen is released. The relative amounts of oxygen and potassium chloride (KCl) will be measured by determining the amount of residue (KCl), which remains after heating. The decrease in weight after heating will represent the amount of oxygen originally present in the sample.
To test that the residue is potassium chloride it will be dissolved in water, acidified and then tested with silver nitrate (AgNO
3
) solution. If the residue is potassium chloride then the acidified solution will react with the silver nitrate solution producing silver chloride (AgCl), which will appear as a white precipitate.

Apparatus:
1. Porcelain crucible with lid *
2. Tongs
3. Clay triangle *
4. Analytical balance *
5. Ring clamp *
6. Spatula
7. Fisher burner and lighter *
8. Three large test tubes
9. Test tube rack
10. Desicooler *
Solutions:
1. Dilute nitric acid (HNO
3
) solution
2. Silver nitrate solution * (about 0.1 M)
Solids:
1. Solid potassium chlorate * (KClO
3
)
2. Solid potassium chloride (KCl)
Precautions:

Wear safety glasses when heating the crucibles and when adding nitric acid in Part B.

Heat the sample slowly and carefully to avoid splattering molten (liquid) material and also to avoid getting poor experimental results.
* View photos on course website

Procedure:
Part A - Decomposition of Potassium Chlorate Reaction *
1. Obtain a clean crucible and lid from your instructor. 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. Record the mass of the crucible and the KClO
3
to four decimal places on the data sheet.
Use an analytical balance and weigh approximately 2.5 +/- 0.1 grams of solid potassium chlorate (KClO
3
) into the crucible.
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.
* View photos on course website

Procedure:
Part B - Examination of Residue *
1. Place 3 test tubes in a test tube rack. Label the test tubes as #1, #2, and #3.
2. Use a spatula to transfer ½ a spatula tip of the following samples into each of the test tubes.
Test tube
#1
#2
#3
Sample
Potassium chloride
Residue from crucible
Potassium chlorate
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 (HNO
3
) and mix. Record your observations on the data sheet.
5. To each test tube add about 5 drops of AgNO
3
and mix. Record your observations on the data sheet.
NOTE: If silver chloride (AgCl) is present, it will appear as a white precipitate.
* View photos on course website

Datasheet:
Part A: Decomposition of Potassium Chlorate Reaction
1. Mass of empty crucible
2. Mass of crucible and KClO
3
3. Mass of crucible and residue
(after 1st heating)
4. Mass of crucible and residue |
(after re-heating)
5. Mass of KClO
3
used
6. Mass of residue which remains
7. Mass of oxygen released
Calculation of % oxygen (experimental):
Calculation of % oxygen (theoretical):

Datasheet:
Part B: Examination of Residue
Record
Observations of:
Test tube #1
Potassium Chloride
(KCl)
Solids
Test tube #2
Residue
Test tube #3
Potassium Chlorate
(KClO
3
)
After addition of
5 mL of water
After addition of
3 drops of HNO
3 solution
After addition of
5 drops of AgNO
3 solution

Postlab Questions:
1. Write the balanced chemical equations and word equations for all the chemical reactions, which occurred in this experiment.
2. Calcium chlorate, Ca(ClO
3
)
2
, when heated also releases oxygen leaving the metal chloride as the residue. Write a balanced chemical equation and word equation for this reaction.
3.
What would be the effect of insufficient heating on your experimental result? Explain fully.