September 12, 2003
Experiment: To Determine the Empirical Formula of Magnesium Oxide
The empirical formula is the simplest and lowest whole number ratio of the different atoms in a sample of compound. To work out the empirical formula, the value of moles of the different atoms in a compound is needed. Mole is just simply a unit used to measure the amount of atoms, just like how the unit “dozen” is used to measure things such as eggs. One mole is 6×10^23 atoms and this number is called the Avogadro number. The mole can be also defined as the number of atoms in exactly 12g of Carbon.
In this experiment, the magnesium will be heated and this magnesium will react with the oxygen in the air to form magnesium Oxide.
- An empty crucible and its lid were weighed on an electrical balance. The mass of the crucible and its lid were recorded.
- The surface of the magnesium ribbon was scraped with emery paper to remove any magnesium oxide on the surface.
- The magnesium was coiled and placed in the crucible. Then the lid was replaced on the crucible, and the mass of the crucible with lid containing the magnesium was found. The mass was recorded.
- The safety goggle was worn. Then apparatus was setup as shown in the diagram. The magnesium was first heated with a small blue flame and after a several minutes with a roaring flame.
- Every few minutes, the lid of the crucible was lifted carefully with a tong to see if the magnesium had started to burn and also to left some air enter the crucible to make the experiment more efficient.
- When the magnesium had completely reacted, the crucible was heated strongly without the lid on for few minutes. Then, the crucible was placed on gauze for it to cool down.
- When the crucible cooled down so that one was able to hold it, the crucible with lid containing the magnesium oxide was measured. The mass was recorded.
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- White fume was coming out of the crucible, when the lid was lifted.
- When the magnesium was first heated, the magnesium was glowing slightly orange, but it turned into a strong glow later.
- Short after the magnesium had started to be heated, the magnesium turned black. However, as the reaction was beginning to finish, the magnesium became an ashy white color.
Mass of magnesium:
Mass of crucible and lid with magnesium – Mass of empty crucible and lid
35.74g – 35.40g
Mass of oxygen:
Mass of crucible and lid with magnesium oxide – Mass of crucible and lid with magnesium
35.94g – 35.74g
Moles of magnesium:
Mass of magnesium
Ar of magnesium
= 0.014167 (6 D.P)
Moles of oxygen:
Mass of oxygen
Ar of oxygen
Mole ratio of magnesium : oxygen:
Magnesium : oxygen
0.0141... : 0.0125
1.1333… : 1
1 : 1
Therefore the empirical formula is…….
The empirical formula of magnesium oxide found from this experiment is MgO. The accepted empirical formula of magnesium oxide is MgO, so the formula that was found out from this experiment was identical to the accepted formula. However, the mole ratio of magnesium : oxygen was not exactly 1 : 1, but 1 : 1.1333…. The value was rounded down to 1. So, the empirical formula of magnesium oxide obtained from this experiment is not exactly MgO. There are several possible sources of error that made the value differ from the accepted value. One possible source of error is that there might have been some magnesium left that had not reacted with the oxygen. This made the mass of oxygen that reacted with the magnesium low, which made the mole of magnesium higher than the accepted value. This can be prevented by making sure that all the magnesium has reacted with the oxygen. When the magnesium has all converted into white magnesium oxide powder, the reaction has completed. This is because when only the product is present, it means that all the reactants had been used up for the reaction. Another way to prevent this error is to make sure that all the surface of the magnesium is touching the air. Unless all the magnesium is able to contact with the air that contains oxygen, not all of the magnesium will react. An additional possible source of error is that some white fume magnesium oxide escaped from the crucible. This error can be prevented by performing the experiment in a fume cupboard. The most significant source of error is that some magnesium possibly reacted with Nitrogen in the air to form Magnesium Nitride. Since 78% of air is made up of Nitrogen, it is highly possible that some Magnesium reacted with the Nitrogen. This made the mass of oxygen that reacted with the magnesium low, which made the mole of magnesium higher than the accepted value. This error can be prevented by burning the magnesium in pure oxygen, not air; although, this is very difficult to do in reality. One way the experiment can be improved is to perform the experiment several times and record the average result so that a more accurate result can be obtained.