The Mol and Avogadro's Number

The gram relative weight of a substance is defined as the relative weight expressed in grams. For example, if something has a relative weight of 2 then its gram-relative weight would be 2 grams; and if you were to mass out one gram-relative weight of this substance you would be massing out 2 grams of it. Likewise, if another substance had a relative weight of 32, like the sulfur atom, 32 grams of this substance would be equal to one gram-relative weight of it. Based upon this definition of gram-relative weight it is possible to complete line three of the chart below.

1. Set up a system of relative weights for the following objects: tack, clip, bead. This can be accomplished by:

  1. Finding the actual mass of each object
  2. Choosing one of the objects as a standard for comparison and giving it an arbitrary relative weight.
  3. Use ratio and proportion to find the relative weight of each of the other objects.

2. Based upon the definition of g-relative wt. in the discussion above fill in line 3 of the chart.
3. Mass out one g-relative weight of each of the three substances and fill in line four of the chart by counting the number of "atoms" present.



actual mass (G)



relative weight



gram-relative weight (G)



"atoms" in 1 g-rel. wt.




1. What conclusion can you come to about the usefulness of the definition given for "gram-relative weight"?
2. Compare your results to those of other students.
3. What determines the value arrived at in line four of the chart?

Teacher Notes
Prior to this exercise I have extensively discussed with my students the difference between atomic mass number, atomic number, actual atomic mass, and the atomic weight (or atomic mass) of the elements. (Note: Keep emphasizing that atomic weights are relative values!!)

The value arrived at in line four of the chart should be the same. However, due to the fact that all clips, beads, and tacks do not have the same mass (isotopes??), results might vary by plus or minus one. This fact should be pointed out to students. I have found that the error is reduced by finding the average of 10 or 20 items of each object and using this as the actual mass in line one of the chart. The error can be almost eliminated if you have the students sort the objects first keeping only those to work with which are about the same mass.

Today the accepted value of Avogadro's number to six significant digits is 6.02257 X 1023. During the discussion following the exercise it should be brought up that Avogadro was not the first person to measure the number of atoms in one mol only the first person to realize that equal "amounts" of substances contained the same number of atoms. The value was first measured to be 6.06 X 1023 by the American chemist Robert Millikan. The term "mol" (or mole) is simply a short hand version of describing a number (6.02 X 1023) just like the word "dozen" describe the number 12.

This exercise is most beneficial to those students with a weak background in math. It allows them to see that if you have a system of relative weights, one gram-relative weight of substance "A" contains the same number of particles as one gram relative weight of substance "B". This fact is true regardless of the actual mass of each substance. Students with a stronger math background can actually prove that this fact must be true.

Questions? Comments??