Relative Masses

John Schwartz                  Senn Metropolitan Academy
                               5900 N.Glenwood Ave.
                               Chicago, IL 60660
     The purpose of this mini-teach is to demonstrate the meaning of relative mass 
and to set up a scale of relative masses. 

      Five Sheet Metal Screws,
      Five Bolts, 3/4 in.
      Five Bolts,1/2 in.
      Five Bolts, 1 1/2 in.
      Five Tacks,
      Five Hex Nuts,
      Five Square Nuts,
      Five Roofing Nails
      Eight Balances,
  and Sixteen Styrofoam cups.  
     *Optional: Calculators.  
     Have each group weigh each of the eight items between 12:30 P.M. and 1:00 
P.M.  Each group then brings its statistics to class for mini-teach. I have 
assumed that my students have already been introduced to the atomic theory.  John 
Dalton is credited with formulating the basic atomic theory that we use today Its 
major parts are the following: 
     1. All matter is made up of small particles called atoms.  
     2. Atoms of the same element are chemically alike.                      
     3. Each kind of atom has its own definite average mass.
     4. Atoms of most elements can enter into combinations called compounds.
     5. Atoms are not subdivided in compounds.

     Can any of these parts of the atomic theory be used to set up a table of 
elements? Yes, we can use points one and three: atoms are very small and each atom 
has its own relative mass. Since atoms are very small, we cannot weigh them 
directly, but we can weigh them indirectly and obtain their relative masses. 

     I have eight items which I will call elements A, B, C, D, E, F, G and H. You 
will tabulate and then enter your results on data/worksheet #1. Then, bring your 
results downstairs to room 118 to continue the mini-teach. 

     Then, the teacher will lead a discussion asking the following questions: What 
is relative mass?  What is a standard?  What are the characteristics of a 
standard? How do you choose a standard-the smallest, the largest? We will come to 
a consensus of opinion: choose one as a standard. Calculate the relative mass of 
each in comparison to the standard. Construct a table of relative masses using the 
data collected by the eight groups.  
     Finally, we will use the concept developed and discuss the derivation of  the 
atomic scale historically: first, hydrogen; then, oxygen; and finally, carbon-12 
as standards.  

      Pass out1 Experiment A-23: Relative Masses of Atoms[Copper/Oxygen]. Also 
pass out xerox copies of the periodic scale. Show students which figures are the 
relative masses of each element. Give the mass of the test tube, the mass of the 
test tube and CuO, and the mass of the test tube and Cu. Use molar quantities of 
each as specified in the reaction given in experiment 23. Do not tell the students 
though. Compute the ratio of copper/oxygen. Use the periodic chart which you were 
given to compute the copper/oxygen ratio. Compare your results for each. Possibly 
at this point you could look at your results and tell the students that you used 
molar quantities, how you computed these molar quantities, how they can compute 
these molar quantities themselves, and why chemists use molar quantities. 
     1Atkinson, Gordon and Henry Heikkinen.  Reactions And Reason.  New York: 
Harper & Row, 1978, pp. 40-41.  
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