We pass dry hydrogen over heated copper(II) oxide to remove the oxygen, leaving copper. From the mass of copper and the mass of oxygen removed, the mole ratio gives the empirical formula, which works out as CuO.
Where magnesium oxide is made by combining a metal with oxygen, copper(II) oxide is studied the other way round: a metal oxide is reduced to the metal, and the oxygen removed is measured. This experiment therefore tests the same mole-ratio reasoning from the reduction side, and it is a standard practical in The Mole Concept, Chemical Formula and Equation chapter. This guide sets out the method, the observations, and the Paper 3 skills the practical assessment rewards.
Aim
To determine the empirical formula of copper(II) oxide by reducing it with dry hydrogen and finding the ratio of the number of moles of copper atoms to the number of moles of oxygen atoms in the oxide.
Apparatus and materials
- Dry copper(II) oxide (black powder)
- Combustion tube with a small hole near one end
- A supply of dry hydrogen gas
- Bunsen burner
- Retort stand, boss and clamp
- Electronic balance
- Porcelain boat or a suitable holder for the oxide
- Heat-resistant mat
Procedure
- Weigh the empty porcelain boat and record its mass.
- Place a quantity of dry copper(II) oxide in the boat, weigh the boat with the oxide, and record the mass.
- Put the boat inside the combustion tube and clamp the tube horizontally, with the small hole pointing upwards near the far end.
- Pass a stream of dry hydrogen through the tube to flush out all the air.
- Test the hydrogen for purity at the hole; only when it burns quietly, showing the air has been removed, is it safe to continue.
- Keeping the hydrogen flowing, heat the copper(II) oxide strongly and light the hydrogen escaping at the hole.
- Continue heating until all the black oxide has changed to brown copper.
- Stop heating but keep the hydrogen flowing until the tube and boat are cool, so the hot copper is not re-oxidised by air.
- Weigh the boat and copper, then heat again in hydrogen, cool in hydrogen and reweigh until the mass is constant, and record the final mass.
Expected observations
The black copper(II) oxide gradually changes to brown, shiny copper as it is heated in hydrogen. Colourless droplets of water condense on the cooler parts of the combustion tube, because the hydrogen combines with the oxygen removed from the oxide. The mass of the solid after reduction is less than the mass of copper(II) oxide at the start, because the oxygen has been removed.
Inference and conclusion
The decrease in mass is the mass of oxygen that was combined with the copper in the oxide. The mass of copper is the mass of the residue, and the mass of oxygen is the loss in mass. Dividing each mass by its relative atomic mass gives the number of moles of copper atoms and of oxygen atoms; the simplest whole-number ratio gives the empirical formula. The ratio works out as one mole of copper to one mole of oxygen, so the empirical formula of copper(II) oxide is CuO, and the reduction follows CuO + H₂ → Cu + H₂O.
Science process skills (Paper 3 style)
Making a hypothesis. In copper(II) oxide, copper and oxygen are combined in a fixed ratio, so a fixed mass of copper is always combined with the same mass of oxygen.
Identifying variables. The manipulated variable is the mass of copper(II) oxide used; the responding variable is the mass of copper obtained; the controlled variables include a steady supply of dry hydrogen and heating to constant mass.
Tabulating data. Draw a table for the mass of the boat, the mass of the boat and copper(II) oxide, and the mass of the boat and copper, so the mass of copper and the mass of oxygen can be found by subtraction.
Making inferences. Explain why the hydrogen flow is kept on while the tube cools: hot copper would react with oxygen in the air and gain mass, so keeping hydrogen flowing protects the result.
Operational definition. The reduction is operationally defined as complete when the solid is fully brown and repeated heating no longer changes its mass.
Safety precautions
- Remove all the air with hydrogen and test the gas for purity before heating, because hydrogen and air together form an explosive mixture.
- Keep the hydrogen supply away from other flames, because hydrogen is highly flammable.
- Wear safety goggles and handle the hot tube with a clamp, to avoid burns.
- Work in a well-ventilated area, so escaping hydrogen does not build up.
Common errors
- Heating before the air is removed. This risks an explosion; always flush with hydrogen and test for purity first.
- Stopping the hydrogen too soon. If the flow is stopped while the copper is hot, air re-oxidises it and the mass rises, spoiling the result.
- Not reducing to constant mass. If some oxide is unreduced, the mass of oxygen removed is too small and the ratio is wrong.
- Using damp copper(II) oxide. Moisture adds mass that is not part of the oxide; use dry oxide and dry hydrogen.
How our teachers use this experiment
In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we pair this with the magnesium oxide experiment so students see the mole ratio from both directions, oxygen added and oxygen removed. A student who can explain the purity test and why hydrogen keeps flowing during cooling can also write the CuO working cleanly. Because empirical-formula and redox ideas recur across SPM Chemistry and this is a classic Paper 3 practical (Paper 3 is a practical test assessing science process skills), mastering the safety routine and the calculation here pays off in both Paper 2 and Paper 3.
Worried about Paper 3?
We coach the practical skills one to one, from hypotheses to graphs and inferences.
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