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Composition of Magnesium Oxide

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A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

Composition of Magnesium Oxide
This experiment is designed to calculate the formula of magnesium oxide formed when magnesium is heated in a crucible.

Method:
1. Prepare a table for your results.
2. Clean the magnesium ribbon with sandpaper and coil it tightly.
3. Weigh the crucible and lid. Then add the magnesium, replace the lid and weigh it again. Record the masses in your table.
4. Place the crucible on the pipe-clay triangle on the tripod. Heat until the base of the crucible is red hot.
5. Using the tongs, lift the lid very briefly to let air in. Try to prevent any smoke from escaping.
6. Repeat step 5 until all the magnesium has burnt to magnesium oxide.
7. Continue heating with the lid off for about 1 minute, to make sure that all the magnesium has reacted. Then let the crucible cool.
8. Weigh the crucible, lid and magnesium oxide. Record the result in your table.

Results:
1. Find the mass of:
a) magnesium used
b) magnesium oxide formed
c) oxygen added.
2. Use your data to in 1, to calculate the amount of magnesium and and the amount of oxygen in your product.
3. Using your answers in 2, calculate the percent composition of magnesium and oxygen in magnesium oxide.
4. The actual % composition by mass of magnesium oxide is: 60% magnesium, 40% oxygen. Comment on any differences between these values and the values you obtained in 3.
5. Using your answers in 2, determine the empirical formula of magnesium oxide.
(If you are not sure how to do the above calculations, refer to the following video on how to calculate the empirical formula magnesium oxide).



Questions
1. Why is it useful to have more than one set of results to determine the empirical formula of magnesium oxide.
2. What are likely to be the main sources of error in this experiment?

Answers
  • Show Solutions
    1. Having several sets of results allow us to get the average and helps to eliminate errors present in any one set of results.
    2. Incomplete reaction of all the magnesium. Loss of magnesium oxide as a white smoke when lid is lifted.
Empirical Formula and Molecular Formula
The difference between empirical formula and molecular formula.
How to write the empirical formula for a compound when you are given the molecular formula?
Molecular formulas tell you how many atoms of each element are in a compound, and empirical formulas tell you the simplest or most reduced ratio of elements in a compound. If a compound's molecular formula cannot be reduced any more, then the empirical formula is the same as the molecular formula. Many compounds with different molecular formula have the same empirical formula.

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