1. IC5.3.3 Masses from experimental data © Oxford University Press Masses from experimental data

2. IC5.3.3 Masses from experimental data © Oxford University Press  The formula for copper oxide is CuO.  The formula contains one Cu and one O.  You may think that 50% of the mass of copper oxide would be copper and 50% oxygen. But this is wrong.  The atoms of different elements usually have different masses, so this needs to be taken into account.

3. IC5.3.3 Masses from experimental data © Oxford University Press You can use experimental data to work out the mass of copper and oxygen in copper oxide.

4. IC5.3.3 Masses from experimental data © Oxford University Press Copper oxide can be reduced to copper using hydrogen as shown.

5. IC5.3.3 Masses from experimental data © Oxford University Press Here is the equation for the reaction: copper oxide + hydrogen  copper + water CuO + H 2  Cu + H 2 O The copper oxide must be hot for this to work. The excess hydrogen has to be burnt off to make the experiment safe.

6. IC5.3.3 Masses from experimental data © Oxford University Press Mass (g) Aempty reduction tube31.9 Breduction tube + copper oxide34.9 Creduction tube + copper34.3 You measure the mass of  the empty tube  the tube containing copper oxide  the tube and contents after the reaction. Here are some example results.

7. IC5.3.3 Masses from experimental data © Oxford University Press CuO + H 2  Cu + H 2 O  mass of copper oxide (B – A) = 34.9 – 31.9 = 3.0 g Now you can find the masses of copper and oxygen in the original copper oxide:  mass of copper (C – A) = 34.3 – 31.9 = 2.4 g  mass of oxygen (B – C) = 34.9 – 34.3 = 0.6 g Mass (g) Aempty reduction tube31.9 Breduction tube + copper oxide34.9 Creduction tube + copper34.3

8. IC5.3.3 Masses from experimental data © Oxford University Press You can now calculate the percentage mass of each copper and oxygen in copper oxide. Percentage mass of copper = percentage mass of copper = mass of copper × 100% mass of copper oxide 2.4 g × 100% = 80% 3.0 g

9. IC5.3.3 Masses from experimental data © Oxford University Press So the percentage mass of copper in copper oxide is 80%. Can you think of two ways to calculate the percentage mass of oxygen in copper oxide?

10. IC5.3.3 Masses from experimental data © Oxford University Press  You could use the same method that you used to find the percentage of copper: percentage mass of oxygen =  Or you could subtract the percentage mass of copper from 100% to find the percentage mass of oxygen: 100% – 80% = 20% 0.6 g × 100 = 20% 3.0 g