Practical: Determine the Formula of Magnesium Oxide
Aim:
To determine the empirical formula of magnesium oxide by combustion of magnesium
Diagram:
Finding the empirical formula of magnesium oxide
Method:
- Measure the mass of the crucible with the lid
- Add a sample of magnesium into the crucible and measure the mass with the lid (calculate the mass of the metal by subtracting the mass of the empty crucible)
- Strongly heat the crucible over a Bunsen burner for several minutes
- Lift the lid frequently to allow sufficient air into the crucible for the magnesium to fully oxidise without letting magnesium oxide smoke escape
- Continue heating until the mass of the crucible remains constant (maximum mass), indicating that the reaction is complete
- Measure the mass of the crucible and its contents (calculate the mass of metal oxide by subtracting the mass of the empty crucible)
Working out the empirical formula:
Mass of metal:
Subtract the mass of the crucible from magnesium and the mass of the empty crucible
Mass of oxygen:
Subtract the mass of the magnesium used from the mass of magnesium oxide
Step 1 – Divide each of the two masses by the relative atomic masses of the elements
Step 2 – Simplify the ratio
magnesium | oxygen | |
mass | a | b |
moles | a / Ar | a / Ar |
x | y |
Ratio = x : y
Step 3 – Represent the ratio in the form ‘MxOy‘ E.g, MgO