Sunday, 4 January 2015

3.5.2 - Laboratory Activity : Determining the empirical formula of magnesium oxide

Laboratory Activity 3.5.2:
Determining the empirical formula of magnesium oxide
Aim: To determine the empirical formula for magnesium oxide
Material:
» Magnesium oxide
» Sand paper
Apparatus:
» Crucible with lid
» Tripod stand
» Bunsen burner
» Electronic balance
» Clay pipe triangle
Procedure and Results:

Procedures
1. The animation below shows the arrangement and the results of the experiment.
2. A 20 cm long magnesium ribbon is polished with sand paper to remove the oxide layer on its surface.
Precautionary steps : The magnesium tape is polished before heating to remove any magnesium oxide that is formed on its surface.
3. The magnesium ribbon is rolled into a loose coil.
4. An empty crucible with its lid is weighed and the weight is recorded.
5. A piece of dry asbestos paper is placed in the crucible followed by the coiled magnesium ribbon. The crucible with its contents is weighed again. The weight is recorded.
6. The crucible is placed on a clay triangle on a tripod and is heated strongly. The lid is lifted at intervals to allow the oxygen from the air to enter for the combustion of magnesium from escaping by closing the lid immediately after it is lifted.
Precautionary steps: The crucible lid is lifted occasionally to allow the oxygen from the air to enter for the complete combustion of magnesium.
7. When the magnesium is completely burnt, the crucible is cooled and is weighed again together with its lid. The weight is recorded.
Precautionary steps: As soon as magnesium burns, the crucible is closed to avoid the white magnesium oxide fumes from escaping, thereby causing a decrease in mass of oxide formed.
8. The heating, cooling and weighing process is repeated until a constant mass is obtained.

Results
Mass of crucible + lid
= a g
Mass of crucible + lid + magnesium
= b g
Mass of crucible + lid + magnesium oxide
= c g
Calculation :

Mass of magnesium obtained = (ba) g
Mass of oxygen that combines with magnesium = (cb) g
[Relative atomic mass: O, 16; Mg, 24]

Element Mg O
Mass (ba) (cb)
Number of moles b - a 24 c - b 16
Simplest ratio x y

The ratio of Mg : O = x : y
Thus the empirical formula is MgxOy
Discussion :

The purpose of repeating the heating, cooling and weighing process is to ensure complete reaction of the magnesium oxide.
Conclusion:

The empirical formula of magnesium oxide is MgO.


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