Relative Atomic Weight

Purpose:
The purpose of this experiment is to calculate the percentage composition of a compound and from this determine the relative atomic weight of the unknown element in the compound. Since this is a quantitative experiment, all measurements recorded in your data table to the proper number of significant digits. The compound to be used will be designated by the formula MO3, where M will be used to represent the unknown element and O will be oxygen. Since this compound is sensitive to heat it can be easily decomposed to element M and oxygen. The oxygen, being gaseous, will escape and leave the substance M behind. The balanced equation is: 2 MO3 ------> 2 M + 3 O2

Procedure:
Mass a clean dry test tube and record to the nearest milligram (0.001 G). Place about 1 to 2 grams of the compound to be decomposed in the test tube and record its mass. Clamp the test tube to a pole and heat. The solid will melt and begin to bubble as it decomposes. The temperature of the flame should be continually adjusted so that the liquid "boils" at a nice even rate. Do not let the liquid "boil" too fast. Continue heating until the liquid turns to a solid. Then heat very strongly for at least five minutes. (During this time the solid may remelt but will not boil, this is normal.) Cool the test tube and record its mass. {Note: During cooling the test tube may crack. If this occurs do not reheat the test tube!! Consult your instructor if this happens.}

Reheat the test tube strongly for 5 minutes, cool and once again determine its mass. These last two massings should agree to within plus or minus 0.005 grams. This is called heating to a constant mass. If they do not agree continue heating the solid until two successive massings agree to within plus or minus 0.005 grams. Heating to a constant mass is the only way to insure that the reaction is complete.

Questions
1. Determine from your data the mass of element M and the mass of oxygen in the compound.
2. Determine the percentage of M and oxygen in the compound.
3. Determine the relative weight of M. (Assume the atomic weight of oxygen is 16.00.) Make sure your answer has the correct number of significant digits for your data.


Teacher Comments
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This is a nice quantitative experiment to show students how the relative atomic weight of an isotopic mixture of an element was historically determined. To four significant digits the atomic weight of oxygen can be taken to be 16.00. For the compound MO3, I used either potassium chlorate or sodium chlorate. Students need to be instructed in the proper use of these type of compounds!! The atomic weight of M is not really an an element but either KCl or NaCl, but students need not know this. Make sure that all glassware is clean and that only uncontaminated chlorate compounds are used. Even though chlorate compounds are dangerous, I have not had a single incident in the many years that I have used this lab. If the test tube cracks while cooling after the first heating, I normally let the students use the data from the first massing as the "constant mass".
 
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