F.6/7 Chemistry Manual: Empirical Formula of Black Copper Oxide
Objective: To determine the empirical formula of black copper oxide by weighing method
Size of group: Two students
Introduction:
In this experiment, a certain quantity of black copper oxide is weighed. Town gas is allowed
to pass the heated copper oxide in a boiling tube. Copper oxide undergoes reduction and the
oxygen is removed and copper is left in the tube. Mass of the product, Cu is measured and
mass of oxygen in the compound is calculated. Hence number of moles of copper and oxygen
are determined and the mole ratio of the elements can be determined. By using this ratio the
empirical formula of copper is estimated.
-----------------------------------------------------------------------------------------------------------------Theory: Give principle/reason to explain how is the formula of copper oxide determined
in this experiment. The following questions should be useful to you.
1. Compare the reactivity of copper with other metals in the metal reactivity series.
2. What is the method employed in this experiment to find the masses of copper and oxygen
for the copper oxide in the boiling tube?
3. Suppose the mass of copper oxide strip is m1, mass of the product is m2, show, how do you
find out the mole ratio of copper and oxygen in the copper oxide?
4. How can you find out the empirical formula from the mole ratio?
----------------------------------------------------------------------------------------------------------------Chemicals:
Black copper oxide
Additional Materials:
Boiling tube fitted with a two-hole stopper. The stopper should be fitted to town gas supply, balance
readable 2 decimal places, stand and clamp, long wooden splint
Safety Precaution
Use of Town gas: It is explosive if town gas mixes with air/oxygen. Consult with your teacher
whenever necessary.
Procedure
Record your results immediately after you get the readings. Wear goggle in the course of
the practical
excess town
gas burning
tube A
clamp
boiling tube
black copper
oxide
town gas
supply
rubber tubing
heat
1. Weigh the dry boiling tube accurately (without the stopper and tubings.). Record the
mass
2. Put 4 spatula measures (about 2 g) of pure, dry copper oxide into the boiling tube.
Weigh the boiling tube and the copper oxide. Record the mass.
3. Refer to the set-up diagram, shake the boiling tube to spread the copper oxide into a
thin layer (not too near the mouth of tube). Put the stopper (with tubings) in place and
clamp the boiling tube horizontally. Connect the rubber tubing to the town gas supply.
Check that tube A is pointing upwards.
Mole2: Formula of magnesium oxide /p.1
4. Light a long wooden splint and hold it in one hand. With the other hand, turn on the gas
supply to a gentle flow (with the gas tap only half-open). Let the gas pass for about 4
seconds. Then carefully light the gas coming out of tube A. Adjust the gas tap until the
flame is about 5 cm high. Keep this flame on throughout the experiment. If it goes out,
light the escaping gas again at once.
DON’T LEAVE THE SET-UP WITHOUT ATTENTION!
5. Light a Bunsen burner. Adjust the flame to a medium non-luminous one. Place the
flame under the boiling tube at the position of the copper oxide.
6. To heat all the solid powder, keep moving the Bunsen flame to and fro along the tube
for a few minutes, until there is no further colour change in the solid.
7. Turn off the Bunsen flame used for heating. Do not shut off the town gas passing
through the boiling tube.
8. Wait for about 10 minutes. When the boiling tube is quite cool (below 70oC), shut off
the gas supply. Remove the stopper and the tubings. (Caution: End of tube A may be
still quite hot.) Weigh the tube and copper.
Data and Results
Mass of boiling tube + black copper oxide
Mass of boiling tube + copper
Mass of boiling tube
Mass of boiling tube + copper
Mass of oxygen
=
=
=
=
=
Calculation
1.
2.
Calculate number of moles of copper and oxygen in black copper oxide.
Calculate relative number of moles of copper and oxygen in the copper oxide. Hence find out the
empirical formula of black copper oxide.
Questions for discussion
1. List safety precautions for this practical.
2. Knowing that the empirical formula of black copper oxide is CuO, calculate percentage error for
your results. Do you accept your results?
3. List all sources of experimental error.
4. If there is copper oxide unreacted in the crucible, suggest a method to confirm the presence of this
unreacted reactant.
5. Explain step 7.
The Report: Hand in your report to Mr. Lai (The Preparation Room) before 5:00 p.m.
Mole2: Formula of magnesium oxide /p.2