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

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:

Mass of metal:

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

Mass of oxygen:

• Subtract 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 / A_r              b / A_r

    = x                   = y

Ratio                x          :           y

Step 3 – Represent the ratio into the form ‘M_xO_y‘ E.g, MgO