Introduction:
As you have
seen, the formula of a compound tells us the number and kind of elements that
make-up that compound. This is known as the chemical composition of a compound.
With a simple calculation using the periodic table or experimental data, the
relative percentage of the elements that make-up a compound can be determined.
This is known as percent composition. Percent composition of each element in a
compound must add up to 100%. The formula to calculate the percent composition
of an element by mass is:
%mass of
element E = Grams of element E x
100
Grams of compound
In this experiment, a chemical
reaction is going to take place that changes potassium chlorate into potassium
chloride and oxygen gas. The oxygen gas will be released from the compound as a
gas escaping through the open end of the test tube. By knowing the mass of the
compound before heating, and the mass of the compound after heating the mass of
the escaped oxygen can be indirectly determined. At the same time, a
theoretical percent composition of oxygen can be calculated and compared to by
the experimentally determined percentage of oxygen.
Objectives:
1. To experimentally determine the percentage of oxygen in potassium chlorate.
2. To calculate the theoretical percentage of oxygen in potassium chlorate.
3. Compare experimental with the theoretical percentage by percent error.
Materials:
Pyrex test tube, balance, clamp, ring stand, Bunsen burner, manganese (IV) oxide, and potassium chlorate
Procedure:
1.
Obtain a clean, dry Pyrex test tube. Add a pinch of manganese (IV) oxide and then mass the test tube
with manganese (IV) oxide to the nearest 0.01 grams.
2. Using weighing paper add between 1.00 grams
and 1.25 grams of potassium chlorate to the test tube.
3. Tap the test gentle to mix the potassium
chlorate with the manganese (IV) oxide.
4. Clamp the test tube to the ring stand at
about 45º angle pointing the opening away from yourself and others.
5. Light the Bunsen burner, and gently heat the
test tube. If sparking occurs than lessen the intensity of the heat.
6. When the solid melts, increase the heating
for 5 minutes. Record time started and ended.
7. Allow the test tube to cool to room
temperature (meaning you can touch it and it is not warm) and mass it.
8. Repeat the heating and massing steps until a
constant mass is obtained. (Meaning mass difference is less than 0.03 grams.
9.
Record all observations of the substance in the
test tube.
Data table:
Create a data table that includes the following columns; Sample and Mass
Your massed samples are: Pyrex test tube with manganese (IV)
oxide, Potassium chlorate, test tube with Potassium chlorate after first
heating, test tube with Potassium chlorate after second heating, and test tube
with Potassium chlorate after third heating.
Analysis:
1.
Calculate the amount of residue of potassium chlorate remaining after
heating.
2.
Calculate the mass of oxygen lost during heating.
3.
Calculate the experimental % mass of oxygen in potassium chlorate.
4.
Calculate the theoretical % mass of oxygen in potassium chlorate.
5.
Calculate the percent error.
Questions:
1.
What is percent composition?
2.
Why was the test tube at a 45 degree angle during heating process?
3.
Write the chemical formulas for the two chemicals used in the experiment.
4.
Why was a pinch of manganese (IV) oxide used in this experiment?
5.
Why was it necessary to repeat the heating of the residue until the mass
was with in 0.03 grams?
Conclusion:
Remember to restate your objectives in the first paragraph and include your results in describing the procedure in the second paragraph. The last paragraph should include you opinion about the effectiveness of the lab procedure meeting the objectives.