3
Most elements found in nature are found combined with other elements as compounds. It is often important to determine the percentage of a particular element in a sample of a substance. An assayer must determine if many compounds in rocks and soil have the desired element in a high enough concentration to make it profitable for mining. Such materials which have sufficient concentrations are called ores.
It is important for a chemist to be able to find the percentage of each element in the compound. Such information is usually necessary to determine the formula of the compound. There are a variety of techniques for separating materials. Frequently elements can be separated by heating the mixture, or heating it with other substances.
In this experiment you will determine the percentage of oxygen in potassium chlorate. When potassium chlorate is heated to high temperatures (about 400 0 C) it decomposes to form potassium chloride and oxygen.
3
2
By massing the substance before and after heating, it is possible to determine the amount of oxygen that is evolved and therefore the percentage of oxygen in potassium chlorate. Sometimes the catalyst manganese dioxide, MnO
2
is added to enhance the decomposition. The catalyst speeds up the decomposition of potassium chlorate but it is not used up in the reaction.
Large test tube tongs ring stand Bunsen burner
Test tube clamp
Potassium chlorate
Manganese dioxide
1. Obtain a large test tube. Weigh it empty. Then add about 0.25 g of MnO
2
catalyst. And weigh it again Usse this mass with the catalyst added as the mass of the empty test tube. Record the mass on your data sheet. (Consider the mass of the MnO
2
as part of the mass of the test tube since it will not change during the experiment.)
2. Now place about 2 grams of potassium chlorate in your test tube and mass it again. Record your mass to two decimal places. (You do not need to have exactly 2.00 grams of KClO
3
. Your mass may deviate by 0.15 gram or so, but record the mass as precisely as the balance will allow.)
3. Spread out the KClO
3
(mixed with MnO
2
) along the bottom third of the test tube. Make sure that the mixture does not extend all the way to the top wall of the test tube when it is lying horizontal. BE SURE THAT YOU ARE
WEARING SAFETY GOGGLES. Suspend your test tube horizontally in the clamp in the ring stand. Heat the test tube carefully and evenly by moving the Bunsen burner back and forth across the length of the mixture in the test tube. Continue this process for about 2 minutes. Be careful since the oxygen released during the heating is very reactive and could cause an explosion. Then heat for another 4-5 minutes. When the reaction appears to be finished allow the test tube to cool.
4. Using your tongs carry the test tube to the nearest balance and measure its mass. Record this number on your data sheet.
5. Return your test tube to the ring stand and reheat it for 2-3 minutes. Allow it to cool and measure its mass again. If it has lost additional mass, heat it again for 2-3 minutes and remeasure its mass. Repeat this procedure until there is no change in mass between successive heatings.
6. Be sure to record your final mass. Allow the test tube to cool and then dispose of the contents in the manner specified by your instructor.

SAMPLE DATA SHEET
1. Mass of the test tube (with MnO
2
only) _____________________
2. Mass of the test tube and KClO
3
_____________________
3. Net mass of KClO
3
_____________________ (line 2- line 1)
4. Mass of Test tube and contents After first heating _____________________
5. Mass of Test tube and contents After first heating _____________________
6. Mass of Test tube and contents After final heating _____________________
7. Total Mass of Oxygen lost _____________________ (line 2- line 6)
8. Mass of residual KCl in the test tube _____________________ (line 6- line 1)
9. Experimental Percent of Oxygen in the Sample _____________________ (line 7 / line 3)
10. Experimental Percent of KCl in the Sample _____________________ (line 8 / line 3)
From your data do the following calculations:
1. Calculate the percentage of oxygen in the sample (experimental value)
Experimental % oxygen = (Total Mass lost by sample) / (Original net mass of the sample)
2. Calculate the percentage of KCl in the sample (experimental value)
Experimental % KCl = 100 - Experimental % oxygen
3. Calculate the percentage of Oxygen in the sample (theoretical value)
Theoretical % oxygen = ( Molecular mass of 3 oxygen atoms) / (Molecular mass of KClO
3
)
4. Calculate the percentage of KCl in the sample (theoretical value)
Theoretical % KCl = ( Molecular mass of KCl) / (Molecular mass of KClO
3
)

5. Calculate the percentage error for oxygen
% error = (Experimental % oxygen) - (Theoretical % oxygen) x 100
(Theoretical % oxygen)
1.
What factors do you suppose are responsible for the difference between the theoretical and the experimental values?
______________________________________________________________________________________
______________________________________________________________________________________
______________________________________________________________________________________
______________________________________________________________________________________
______________________________________________________________________________________
2.
How can you determine that all of the oxygen has been driven off?
______________________________________________________________________________________
______________________________________________________________________________________
______________________________________________________________________________________
____________________________________________________________________________________
3.
Compare your data with that of the other lab groups in your class. How do your results compare? What is the average percent error?
______________________________________________________________________________________
______________________________________________________________________________________
______________________________________________________________________________________
______________________________________________________________________________________
______________________________________________________________________________________