Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations
Copper forms different oxides, such as copper(II) oxide, CuO\text{CuO}CuO, and copper(I) oxide, Cu2O\text{Cu}_2\text{O}Cu2O.
A teacher investigated an oxide of copper using the experimental setup shown below.
This is the method used:
- Weigh empty tube A.
- Add some of the oxide of copper to tube A.
- Weigh tube A and the oxide of copper.
- Weigh tube B and the drying agent.
- Pass hydrogen through the apparatus and light the flame at the end.
- Heat tube A for 2 minutes.
- Reweigh tube A and contents.
- Repeat steps 5 to 7 until the mass no longer changes.
- Reweigh tube B and contents.
- Repeat steps 1 to 9 with different masses of the oxide of copper.
Suggest one reason why step 8 is needed.
Explain why the excess hydrogen must be burned off.
Table 1 shows the teacher's results:
| Mass (g) | |
|---|---|
| Tube A empty | 84.35 |
| Tube A and oxide of copper before heating | 100.35 |
| Tube A and contents after 2 minutes | 98.20 |
| Tube A and contents after 4 minutes | 97.15 |
| Tube A and contents after 6 minutes | 97.15 |
| Tube B and contents at start | 112.40 |
| Tube B and contents at end | 116.00 |
When an oxide of copper is heated in a stream of hydrogen, the word equation for the reaction is: copper oxide+hydrogen→copper+water\text{copper oxide} + \text{hydrogen} \rightarrow \text{copper} + \text{water}copper oxide+hydrogen→copper+water
Determine the mass of copper and the mass of water produced in this experiment.
The teacher repeated the experiment with a different sample of the oxide of copper. The teacher found that this sample produced 1.92 g1.92\text{ g}1.92 g of copper and 0.27 g0.27\text{ g}0.27 g of water.
Two possible equations for the reaction are:
- Equation 1: CuO+H2→Cu+H2O\text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O}CuO+H2→Cu+H2O
- Equation 2: Cu2O+H2→2Cu+H2O\text{Cu}_2\text{O} + \text{H}_2 \rightarrow 2\text{Cu} + \text{H}_2\text{O}Cu2O+H2→2Cu+H2O
Determine which is the correct equation for the reaction in the teacher's experiment. Relative atomic masses (ArA_rAr): H=1\text{H} = 1H=1, O=16\text{O} = 16O=16, Cu=64\text{Cu} = 64Cu=64.