The Formula for Magnesium Oxide.

Amount (in moles) of substance = ___________________

Safety

• Be aware of Bunsen burner flames.
• Use a heat proof mat.
• Use tongs to lift the lid as it will be very hot.
• Keep the working area clear.

Results:

        

The magnesium glowed brightly each time we lifted the lid, however, some smoke was released. A grey-white ashy deposit was left each time we performed the experiment.

Analysing evidence and drawing conclusions:

Conclusion: To find the formula of Magnesium oxide, I must find the ratio of masses of magnesium to oxygen:

RAM of magnesium = 24

RAM of oxygen = 16

Mass 1: magnesium = 0.15g        magnesium oxide = 0.26g

           Oxygen = 0.26– 0.15 = 0.11g

_____________        =        0.11g   = 0.006875 moles

                                          16

        ______________        =        0.15g   = 0.00625 moles

                                          24

Ratio magnesium : oxygen – 1:1 approx

Mass 2: magnesium = 0.16g        magnesium oxide = 0.21g

           Oxygen = 0.21– 0.16 = 0.05g

        ____________        =        0.05g        = 0.003125 moles

                                         16

                                                           .

        ______________        =        0.16g        = 0.0066 moles

                                          24

Ratio magnesium : oxygen – 1:2 approx

Mass 3: magnesium = 0.14g        magnesium oxide = 0.24g

           Oxygen = 0.24– 0.14 = 0.10g

        ____________        =        0.10g        = 0.00625 moles

                                          16

                                                             .

        ______________        =        0.14g        = 0.00583 moles

                                          24

Ratio magnesium : oxygen – 1:1 approx

Each time we lifted the lid, the magnesium glowed brightly because we were letting more oxygen in to react with the magnesium. The smoke which was escaping and the ash deposit were magnesium oxide.

We can see from the graph that the points are mostly clustered between the lines representing MgO and Mg2O. They do not all lie on or around the line for MgO as we might have expected, and none are anywhere near the line for MgO2. The reason for this is that – as none of the points are on the line or below it (except one) – not all the magnesium had reacted, as there would still be some in the centre of the strip (inside it) which would not be reacted. The result which did appear below the line for MgO was anomalous. There is no way that magnesium could react in more than a proportion of 1:1 in atoms, so it must have either been calculated wrongly or more mass must have got in there somehow after the first weighing of magnesium.

The graph and what I have worked out from my own results show my hypothesis was correct. The formula that I am able to deduce from this experiment is that Magnesium Oxide is MgO. The reason for this is that oxygen has six electrons on its outer shell. We know that atoms of a substance try to obtain a “full outer shell of electrons” in the easiest possible way. Oxygen is two electrons short of a full “outer shell”, so it will attract these two needed electrons from any substance it can. Magnesium has two electrons on its outer shell, so the easiest way for a magnesium atom to gain a full outer shell would be to lose those two electrons.

Evaluating evidence:

Evaluation

As we can see from the results, the ratios of masses of magnesium to oxygen are not exactly 1:1, especially the second results set. I remember that in the second experiment especially, I did not let in enough oxygen, which would explain why there was much less oxygen in the outcome. Another reason we did not get a ratio of 1:1, was that we did not manage to trap all the smoke when we lifted the lid to let in the oxygen. This would have let out quite a lot of magnesium oxide out. Another reason was that we did not lift the lid nearly as much as we should have done, so not all the magnesium reacted. Also, some of the magnesium in the second experiment could have reacted with nitrogen in the air, as I did not lift the lid enough for the magnesium to react with the oxygen in the air.

If I were to do the experiment again, I would have a continuous flow of oxygen going in to the crucible or other container like so:

An alternative experiment

This experiment has these advantages. There would be less of a chance of magnesium oxide being lost, or not being reacted. It also has the advantage of not having the risk of the magnesium reacting with the nitrogen in the air.

This alternative way of performing the experiment would be to react a strip of magnesium with nitric acid. This would add the oxygen to the metal indirectly and so has the advantage of the ability to control the vigorousness of the reaction, and so make it safer. The acid would be added slowly until all the magnesium had reacted, and then the product would be placed in a fume cupboard. The contents of the crucible would be heated lightly first to get rid of the water, then the solid residue would be heated very strongly to drive off the brown fumes of nitrogen dioxide. When no more brown fumes could be seen, the crucible would be cooled and weighed.

        

These reactions would have then occurred:

Magnesium (s) + Nitric acid (aq)                  Hydrogen (g) + Magnesium + Nitrogen

       oxide (s)      dioxide (g)

     2Mg          +    2HNO3                             H2           +      2MgO    +  2NO2

Source: Discovering chemistry 2 – M A Atherton & J K Lawrence