Scientific Principles & Theory
Scientific Background: The atmosphere consists of approximately 78% nitrogen, 21% oxygen, 0.9% argon, and 0.04% carbon dioxide. The percentage of oxygen can be measured by reacting a fixed volume of air with an excess of a metal, which removes the oxygen to form a solid metal oxide.
When copper turnings are heated in air, they react with oxygen to form black copper(II) oxide:
Since the copper oxide is a solid, the volume of the gas in the sealed system decreases by an amount equal to the volume of oxygen reacted.
Experimental Variables
Independent Variable
The total volume of air pushed back and forth over the heated metal.
Dependent Variable
The final volume of gas remaining in the syringes after reaction and cooling.
Control Variables
Mass of copper turnings (must be in excess), temperature of system before measuring (must be cooled to room temperature), seals (no leaks).
⚠️ Lab Risk Assessment
| Hazard | Associated Risk | Control Measure |
|---|---|---|
| Hot silica tube and syringes | Thermal burns | Do not touch the hot tube or syringes; allow the apparatus to cool completely before taking the final volume reading. |
| Glass under pressure | Glass breakage / flying shards | Ensure syringes move smoothly; do not over-pressurise the syringes; wear safety goggles. |
Apparatus & Procedure
Required Apparatus
- Two 100 cm³ gas syringes
- Hard glass (silica) delivery tube
- Bunsen burner
- Connecting rubber tubing
- Copper turnings (excess, approx. 5 g)
- Two clamps, stands, and bosses
Step-by-Step Procedure
- Place an excess of copper turnings inside the hard glass silica tube.
- Connect the silica tube between two gas syringes (Syringe A and Syringe B) using rubber tubing.
- Fill Syringe A with exactly 100 cm³ of air. Leave Syringe B completely empty (0 cm³).
- Clamp the syringes and the silica tube securely to stands.
- Heat the silica tube containing the copper turnings strongly using a blue Bunsen burner flame.
- Slowly push the plunger of Syringe A to pass the air over the heated copper into Syringe B.
- Push the plunger of Syringe B to pass the air back over the copper into Syringe A.
- Repeat this passing of air back and forth continuously for 3 to 4 minutes. The copper turnings will turn from red-brown to black as copper(II) oxide forms.
- Stop heating. Allow the entire apparatus to cool down to room temperature. This is essential because gases expand when hot, which would give an incorrect volume reading.
- Push all the remaining gas into one syringe and read the final volume. Record this volume.
- Repeat the heating and cooling cycle until the volume reading remains constant, ensuring all oxygen has reacted.
Fig 1. Laboratory experimental setup for Core Practical 2.14.
Sample Data & Calculations
This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:
| Parameter Measured | Volume of Gas (cm³) |
|---|---|
| Initial volume of air in syringe A | 100.00 |
| Initial volume of air in syringe B | 0.00 |
| Final volume of gas after heating and cooling | 79.00 |
Data Processing & Analysis
- Volume of oxygen reacted = Initial volume - Final volume = 100.00 cm³ - 79.00 cm³ = 21.00 cm³
- Percentage of oxygen in air = (Volume of oxygen reacted / Initial volume of air) * 100
- Percentage of oxygen = (21.00 cm³ / 100.00 cm³) * 100 = 21.00%
Conclusion & Evaluation
Chemical Explanation: Saturated solutions are heavily dependent on temperature. Heating shifts solubility limits, allowing more solute to form coordinate bonds or ion-dipole interactions with solvent molecules. When cooled, the reverse process happens and solute precipitates out.
Experimental Error Analysis
| Error Type & Source | Effect on Final Result | Mitigation Strategy |
|---|---|---|
| Systematic Error Not allowing apparatus to cool to room temperature before taking volume reading |
The gas remains expanded due to thermal energy, so the final volume is read as too high, making the calculated oxygen percentage too low. | Allow the syringes to cool completely (typically 10 to 15 minutes) until the volume reading is constant. |
| Systematic Error Using insufficient copper turnings |
If copper is not in excess, some oxygen will remain unreacted, leading to an underestimation of the oxygen percentage. | Use an excess of copper turnings (about 5 g) so that the copper does not all turn black and unreacted copper remains visible. |
Exam Practice
Describe an experimental method to determine the percentage by volume of oxygen in a sample of air using gas syringes and copper turnings. Explain how you would ensure all oxygen reacts and how you would calculate the final percentage.
View Model Answer & Mark Scheme
Model Answer (6/6 Marks):
- Setup: Place excess copper turnings into a silica tube. Connect the tube to two gas syringes. Set syringe A to 100 cm³ of air and syringe B to 0 cm³.
- Heating: Heat the silica tube strongly with a Bunsen burner flame.
- Passing Gas: Push the plunger of syringe A to pass air over the heated copper. Push syringe B to pass it back. Repeat this continuously for several minutes.
- Ensuring Reaction: Using excess copper ensures all oxygen can react. Repeat the heating/passing cycle until the gas volume stops decreasing.
- Cooling: Allow the entire apparatus to cool to room temperature before recording the final volume, preventing expansion errors.
- Data Processing: Calculate the volume change: volume change = initial volume - final volume. Calculate percentage: percentage = (volume change / initial volume) * 100. For 100 cm³ initial air, the volume change directly equals the percentage.
Examiner Tip:
Explicitly write out the calculation steps and state that cooling to room temperature is necessary because gas volume increases when heated (Charles's Law).