Calculating the empirical formula of a compound


The purpose of this experiment is to calculate the molar ratios of the elements involved in the reaction between Magnesium and Oxygen and use these values to determine the empirical formula.


Magnesium combines with oxygen in the air to form the compound magnesium oxide. There is an increase in mass as the experiment proceeds, implying that matter is being added to the magnesium (oxygen atoms). Therefore, we can assume that any increase in the initial mass of magnesium must be oxygen. As we can calculate both these masses, we can work out the number of moles involved and determine the empirical formula.

Magnesium(s) + Oxygen(g) → Magnesium Oxide(s)


  • 0.30g Magnesium ribbon (coiled into a spiral)
  • Crucible and Lid
  • Bunsen Burner, Heat proof mat, pipe clay triangle.
  • Tongs
  • Balance ±0.01g


  1. Record the mass of crucible and lid to the nearest ±0.01g.
  2. Obtain about 0.30 g (35 mm) magnesium ribbon, spiral the ribbon to fit into the bottom of the crucible ensuring all the surfaces are exposed.
  3. Record the mass of the magnesium ribbon and crucible to ±0.01 g.
  4. Place the crucible securely on the clay triangle. Set the lid slightly off-centre on the crucible to allow air to enter but to prevent the magnesium oxide from escaping.
  5. Place the lit Bunsen burner under the crucible and move the flame back and forth gently heating the bottom of the crucible with the flame for about 1 minute this should prevent the crucible cracking as it expands); then, place the burner under the crucible and heat strongly.
  6. Use the tongs to carefully lift the lid and introduce more oxygen into the crucible (try to avoid letting the magnesium oxide escape the crucible).
  7. Heat until all the magnesium turns into gray-white powder (around 10 minutes).
  8. Stop heating and allow the crucible, lid, and contents to cool. (DO NOT PLACE THE HOT CRUCIBLE ON TO THE BALANCE).
  9. Allow the crucible to cool and then record the mass of the crucible and contents to ±0.01 g.
  10. Heat the crucible and contents as before, for another 5 minutes.
  11. Allow the crucible to cool and then record the mass of the crucible and contents to ±0.01 g.


Mass of Crucible and Lid = ________________________ g

Mass of Crucible, Lid and Magnesium = ________________________ g

Mass of Magnesium = ________________________ g

Mass of Crucible, Lid and Magnesium Oxide = ________________________ g

Mass of Magnesium Oxide = ________________________ g

Mass of Oxygen = (Mass of Magnesium Oxide – Mass of Magnesium)

Mass of Oxygen = ________________________ g

Moles of Magnesium = ________________________ mol

Moles of Oxygen = ________________________ mol

Ratio of moles = Mg _____ : O _____

Remember, when you divide by the smallest number moles you will usually get values very close to whole numbers.

Empirical Formula =


  1. Suggest a reason why you may not get exact integer/whole number relationships?
  2. Write a balanced symbol equation for this reaction
  3. If you were to add water to the magnesium oxide, what pH range would you expect to find?
  4. Suggest a reaction that might occur when Magnesium Oxide dissolves in water.

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