EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记：1.5.7 Practical: Determine the Formula of a Metal Oxide

Aim:

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

Diagram:

Finding the empirical formula of magnesium oxide

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 crucibleMass 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

Mole              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

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 oxideStep 1 – Divide each of the two masses by the relative atomic masses of elementsStep 2 – Simplify the ratio                     Metal            OxygenMass               a                       bMole              a / M_r              b / M_r= x                   = yRatio                x          :           yStep 3 – Represent the ratio into the form ‘M_xO_y‘ E.g, CuO