Determine an Empirical Formula (Edexcel GCSE Combined Science: Chemistry): Revision Note

Determine an Empirical Formula

Aim:

To determine the empirical formula of magnesium oxide by combustion of magnesium

Diagram:

Finding the empirical formula of magnesium oxide involves heating magnesium ribbon very strongly in a crucible. A lid is used to trap any smoke (not shown)

Method:

• Measure mass of crucible with lid

• Add sample of magnesium into crucible and measure mass with lid (calculate the mass of the metal by subtracting the mass of 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 crucible remains constant (maximum mass), indicating that the reaction is complete

• Measure the mass of crucible and contents (calculate the mass of metal oxide by subtracting the mass of empty crucible)

Working out the empirical formula:

Step 1 - Calculate the masses of metal and oxygen:

• Mass of metal:

  • Subtract the mass of the crucible from the mass of the crucible and magnesium

• Mass of oxygen:

  • Subtract the mass of the crucible from the mass of the crucible and magnesium oxide

Step 2 – Divide each mass by the relative atomic mass (A_r)

Step 3 – Simplify the ratio

Step 4 – Write the empirical formula