Determination of the Empirical Formula of Copper Oxide
Experiment Overview
This experiment is designed to determine the empirical formula of copper oxide. Since copper has two common
oxidation states, +1 and +2, there are two possible forms of copper oxide: copper(I) oxide (Cu2O) and copper(II)
oxide (CuO). Copper oxide is insoluble in water but dissolves in acid. A weighed sample will be dissolved in
acid, and an excess of zinc metal will be added to the solution to cause metallic copper to be formed. Excess
zinc can be removed by physical means (fish out the pieces) or chemical means (dissolve in excess hydrochloric
acid). The copper will be filtered and its mass determined. The empirical formula can be calculated from the
masses of copper and oxygen present in the sample.
Materials
Copper oxide solid, ~2.0 g
Hydrochloric acid, HCl, 3 M
Zinc metal, Zn, (mossy zinc)
Procedure
1. Write balanced equations for the steps of this lab. Devise your own procedure. You may check with the
teacher before you begin. Be aware of the following concerns.
2. 3M HCl is hazardous. If you spill it on your skin, rinse with plenty of water.
3. Zinc reacts with the copper ions in solution, but it also reacts with hydrochloric acid to generate hydrogen:
Zn + HCl(aq) → ZnCl2 + H2. Make sure there are no flames around. If the solution is vigorously evolving
gas, you should cover the beaker with a watch glass to prevent splashing.
4. Think about
ƒ How much HCl(aq) do you need, at minimum?
ƒ How will you know when all the copper oxide is gone?
ƒ If it’s taking too long for the copper oxide to react with HCl, how can you speed up the reaction?
ƒ HCl(aq) is HCl(g) dissolved in water. What happens if you boil HCl(aq)? So if you heat the
HCl(aq), do not heat it much above 50°C. Use a glass thermometer.
ƒ Make sure you use enough zinc to react with all the copper ions; if you use excess zinc, it is
relatively easy to remove it later.
ƒ How will you know if all the ionic copper has been converted to Cu(s)?
ƒ When you collect your Cu(s), be sure to rinse off any HCl(aq). When you mass the Cu(s), how will
you know it is dry?
LAB WRITEUP
Prelab
In your notebook, write the introduction to this lab: briefly explain the objective of the lab; be sure to include
balanced equations showing the two possible reaction sequences.
Procedure
Describe your procedure so that a trained chemist could reproduce your results.
Data as required
Observations as required
Calculations
Calculate the weight percent copper in copper oxide that you observed. Determine the empirical formula of the
copper oxide you started with. Calculate the % error in your % copper in copper oxide.
Discussion
What is the result of your experiment—did you start with Cu(I) oxide or Cu(II) oxide? If you had >5% error in
your %Cu, what could have caused your error and how could you prevent this error?
LabCopperOxideEmpForm
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