2. Remove the crucible, using the crucible tongs, and place it on the ceramic pad to cool. After the crucible has cooled to room temperature (this will take about ten minutes), weigh it with its cover on an analytical balance to 0.001 g. Record the weight on the data sheet.
3. Weigh out 0.3g of magnesium and place it into the crucible. Weigh the magnesium, crucible and cover on an analytical balance. Record the weight to 0.001 g.
4. At frequent intervals, remove the burner and slightly lift the crucible lid using tongs, quickly replacing the lid so as to lose as little magnesium oxide 'smoke' as possible. Repeat the process patiently until the magnesium ceases to flare up, then remove the lid and heat strongly to make sure that combustion is complete and all the metal has been converted into oxide. You should not be able to see any shiny metallic surfaces. Allow the crucible to cool then replace the lid and reweigh the whole. Record the mass.
5. Repeat this 5 times so that you will be able to gain an average result.
Fair Test.
To make this a fair test we weighed the empty crucible and lid before adding magnesium. This means that the different weights of the crucibles could be accounted for. This allowing any minor errors in the experiments result to be averaged out. To make this a lot more accurate we carried the test out 5 times.
Before each time we carried out the experiment we cleaned the crucible and lid thoroughly making sure that there was no excess foreign materials of any sort to tamper with the chemical reaction.
Variables.
- The amount of magnesium used in the experiment.
- How long it had been left burning for.
- How much air you let in it when lifting up the lid.
If I were to do this experiment again I would carry it out in a fume cupboard to make sure nothing but pure oxygen will be allowed to come in contact with the burning magnesium.
Potential reasons for error
Hot Magnesium will react with Nitrogen in the air if there is insufficient O2 forming Magnesium Nitrate (Mg3N2).
When magnesium metal is burned in pure oxygen, the only product is magnesium oxide. If a sample of Mg is weighted before and after combustion, then the increase in mass is equal to the mass of oxygen that is combined with Mg. From a knowledge of the mass of Mg and the mass of combined O, the empirical formula of magnesium oxide can be calculated.
There is only one problem with this straight forward approach. It is more usual to burn things in air than in pure oxygen. Air is about 80% nitrogen. So burning Mg in air results not only in the formation of magnesium oxide, but also a small amount of a "byproduct", magnesium nitride.
Not all the Mg was burned. There is not much to do about this, just make sure that you have followed the method correctly and look out for all the signs of the magnesium being present.
I recommend doing this experiment in a fume cupboard so that any products of the combustion do not escape with the gas as you lift up the lid of the crucible to let oxygen in. at lest it will stay in one concentrated area and not around the rest of the room where it won't get used at all.
Safety.
As we were dealing with chemicals and heat during the course of this test it was vital that it was carried out with great caution. To achieve this we:
- Made sure that the work surface was clean and tidy with no unnecessary objects lying around on it.
- Were cautious about burning magnesium as it burns a bright white light, which is possible to blind any one looking straight at it.
- Were careful to wear out protective glasses on at all times.
- Used tongs to move the equipment about as we were dealing with hot things.
- Stood up during the experiment.
- Never left the apparatus unattended and kept a close eye on it at all times noting the changes taking place.
- Put the Bunsen burner on the safety flame when not in use to prevent any mishaps
- Made sure we didn't inhale any 'smoke' when the magnesium was being burned.
Results.
Conclusion.
From my results I can tell that 0.067 molecules of magnesium had combined with 0.069 moles of oxygen, I predict that 1 mole of magnesium will combine with 0.069/0.067 or 1.031 moles of oxygen.
I think that if this experiment had been carried out in a proper laboratory with no interferences you would find that the ratio for this combustion would be very close to 1:1. Which would justify my prediction to be correct.