Practical: Determine the Formula of a Metal Oxide (Edexcel IGCSE Chemistry)

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

   Ratio = x : y

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

Aim:

To determine the formula of copper(II)oxide by reduction with methane

Diagram:

Finding the empirical formula of copper(II)oxide

Method:

• Measure mass of the empty boiling tube
• Place metal oxide into a horizontal boiling tube and measure the mass again
• Support the tube in a horizontal position held by a clamp
• A steady stream of natural gas(methane) is passed over the copper(II)oxide and the excess gas is burned off
• The copper(II)oxide is heated strongly using a Bunsen burner
• Heat until metal oxide completely changes colour, meaning that all the oxygen has been removed
• Measure mass of the tube remaining metal powder and subtract the mass of the tube

Working out empirical formula:Mass of Metal:

Measure mass of the remaining metal powder

Mass of Oxygen:

Subtract mass of the remaining metal powder from the mass of metal oxide

Step 1 – Divide each of the two masses by the relative atomic masses of elements

Step 2 – Simplify the ratio:                     

Step 3 – Represent the ratio in the form ‘M_xO_y‘ E.g, CuO