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Relative Atomic Mass Activity


What is the mass ratio of copper to aluminum?


Tiniest particle of an element that has all the properties of the element. Atoms are composed of protons, neutrons and electrons.
Atomic mass
Average of the masses of naturally occurring isotopes of an element, weighted by abundance.
Atomic mass units (AMU)
Unit to describe the mass of atoms, molecules and subatomic particles. The new name for this unit is Daltons (D).
Atomic number
Number of protons in an atom.
A pure substance made of two or more elements that are chemically combined. The compound will have different physical and chemical properties than the parent elements. The elements always combine in the same ratio or proportions.
The simplest pure substance, composed of just one kind of atom.
Law of Definite Proportions
The elements of a compound always combine in the same ratio or proportions.
Mass number
Mass of an atom in atom mass units (AMU). It is the sum of the number of protons and neutrons.
The smallest particle of a compound that retains the properties of the compound. Consists of two or more elements chemically bonded together.

The Reaction Described

You will be reacting Aluminum (Al) with Copper(II) Chloride (CuCl2).

reactants yields products
Al + CuCl2 Cu + AlCl3

A chemical reaction may be described as:

This is a single-displacement reaction. One atom of a single element (Al) trades places with one atom of another element (Cu).


· Ring stand with ring funnel · filter paper · small beaker (50ml) · large beaker (250ml) · weighing boat · stirrer · Copper(II) Chloride (CuCl2) · Al foil



  1. Safety goggles will be worn at all times during this activity. Consult the copper(II) chloride MSDS external arrow for additional safety information. Ask your instructor if you have any questions or concerns.
  2. Verify that the glassware is clean and dry, the balance is zero'd, and you have the correct chemicals. These extra validity measures ensure that you are really measuring what you think you are measuring.
  3. Place the 50ml beaker on the balance. Record the mass..
  4. Measure 2.00g of Copper(II) chloride directly into the small 50ml beaker. Record the CuCl2 mass.
    Note: Copper(II) chloride is corrosive (a chemical property).
  5. Measure out 0.20g of aluminum foil. Record the mass.
  6. Place the foil in the small 50ml beaker.
  7. Fill the small 50ml beaker about one-half full with distilled water.
  8. Record your observations of the reaction.
  9. You may wish to carefully stir the materials in your beaker to assist the reaction to completion.
  10. Write your class period number and your initials on the filter paper.
  11. Mass the filter paper. Record the mass.
  12. Fold the filter paper in half, then in half again. Open up one side (leaf) of the filter paper to form a closed cone. Place the cone in the funnel.
  13. Place the larger beaker under the funnel to capture the liquid.
  14. When the reaction has completed, pour the contents of the small beaker into the funnel with filter paper.
  15. Use a wash bottle to rinse all of the reaction products into the filter paper.
  16. The next day, when the filter paper and reaction products are dry, mass the filter paper and products. Record.
  17. Calculate the mass of just the reaction products. Record.
  18. Enter your data on the class spread sheet.

Relative Atomic Mass Data:

  1. __________ mass of the small beaker
  2. __________ mass of CuCl2
  3. __________ mass of Al
  4. __________ mass of filter paper
  5. __________ mass of filter paper + products
  6. __________ mass of products
  7. __________ your Cu / Al ratio
  8. __________ class average Cu / Al ratio

Reaction Observations

Relative Atomic Mass Analysis:

  1. What element (the product) has been trapped in the filter paper? __________  
  2. Using the class average Cu / Al ratio, if an Al atom had a relative mass of one, what is the relative mass of one Cu atom? 

  3. Using the class average Cu / Al ratio, what is the relative mass of two Cu atoms?

  4. We have been measuring the relative mass of elements. On a periodic table the mass of an element's atom is measured in AMU. Using a periodic table, record the average atomic mass for Al and Cu:
    • Al _________________ AMU
    • Cu _________________ AMU

  5. Calculate the Cu / Al ratio for the periodic table average atomic masses:

  6. How does the class average Cu / Al ratio compare to the periodic table Cu / Al ratio?

  7. Often groups will have the blue-green residue of CuCl2 or small pieces of Al still in their small beaker, but not both. Explain.

  8. This experiment may be performed using magnesium instead of aluminum. Use the experiment data below:
    1. 0.20g mass of Mg
    2. 2.00g mass of CuCl2
    3. 0.67g mass of filter paper
    4. 1.19g mass of filter paper + Cu
    5. _______ mass of Cu
    6. Calculate the Cu / Mg ratio.


    7. Using the Cu / Mg ratio, if an Mg atom had a relative mass of one, what is the relative mass of one Cu atom?

    8. How does this value for the Cu mass compare with number 2 above?

  9. Explain why this reaction is a single displacement reaction:
    reactants yields products
    Al + CuCl2 Cu + AlCl3