Specification Point 1.31:
Understand how the formulae of simple compounds can be obtained experimentally, including Metal Oxides, Water and Salts containing Water of Crystallisation

Metal Oxides

Formulae of a metal oxide, Edexcel IGCSE Chemistry

The apparatus needed to find the formulae of a Metal Oxide

Method:

  • Measure mass of crucible with lid
  • Add sample of metal into crucible and measure mass with lid (calculate the mass of metal by subtracting the mass of empty crucible)
  • Strong heat the crucible over a Bunsen burner for several minutes
  • Lift the lid frequently to allow sufficient air into the crucible for the metal to fully oxidise without letting magnesium oxide 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 Empirical Formula / Formulae:

Mass of Metal: Subtract mass of crucible from metal and mass of empty crucible

Mass of Oxygen: Subtract mass of metal used from the mass of magnesium oxide

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

Step 2 – Simplify the ratio

                                    Metal            Oxygen

              Mass               x                       y

              Mole              x / Mr              y / Mr

                                     = a                   = b       

              Ratio                a          :           b

STEP 3 – Represent the Ratio into the ‘ Metal O ‘ E.g, MgO

Water and Salts containing Water of Crystallisation

The apparatus needed to find the formulae of crystals

Method:

  • Measure mass of evaporating dish
  • Add a known mass of hydrated salt
  • Heat over a bunsen burner, gently stirring, until the blue salt turns completely white, indicating that all the water has been lost
  • Record the mass of the evaporating dish and contents

Working out Empirical Formula / Formulae:

Mass of White Anhydrous Salt: Measure Mass of White Anhydrous Salt Remaining

Mass of Water: Subtract Mass of White Anhydrous Salt Remaining from the Mass of Known Hydrated Salt

STEP 1 – Divide Each of the Two Masses by the Relative Atomic Masses of Elements

STEP 2 – Simplify the Ratio of Water to Anhydrous Salt

                           Anhydrous Salt      Water

              Mass               a                       b

              Mole              a / Mr               b / Mr

                                     = y                    = x

              Ratio                1          :           x

STEP 3 – Represent the Ratio into ‘ Salt.xH2O ’

Specification Point 1.36:
Practical: Know how to determine the formula of a Metal Oxide by Combustion (E.g. Magnesium Oxide) or by Reduction (E.g. Copper (II) Oxide)

Combustion of Metal Oxides

Metal Oxide: When a metal reacts with and gains oxygen

Combustion of metal oxides, Edexcel IGCSE Chemistry

The apparatus needed to find the formulae of a Metal Oxide by combustion

Method:

  • Measure mass of crucible with lid
  • Add sample of metal into crucible and measure mass with lid (calculate the mass of metal by subtracting the mass of empty crucible)
  • Strong heat the crucible over a bunsen burner for several minutes
  • Lift the lid frequently to allow sufficient air into the crucible for the metal to fully oxidise without letting Magnesium Oxide 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 by subtracting the mass of empty crucible)

Working out Empirical Formula / Formulae:

Mass of Metal: Subtract Mass of Crucible from Metal and Mass of Empty Crucible

Mass of Oxygen: Subtract Mass of Metal used from the Mass of Magnesium Oxide

Step 1 – Divide Each of the Two Masses by the Relative Atomic Masses of Elements

Step 2 – Simplify the Ratio

                                    Metal            Oxygen

              Mass               a                       b

              Mole              a / Mr              b / Mr

                                     = x                   = y

              Ratio                x          :           y

Step 3 – Represent the Ratio into the Form ‘ xO ‘ E.g, MgO

Reduction of Metal Oxides

Reduction of metal oxides, Edexcel IGCSE Chemistry

The apparatus needed to find the formulae of a Metal Oxide by reduction

Method:

  • Measure mass of metal oxide
  • Place metal oxide into a horizontal boiling tube held by a clamp and heat using bunsen burner
  • Heat until metal oxide completely changes colour, meaning that all the Oxygen has been reduced
  • Measure mass of the remaining metal powder

Working out Empirical Formula / Formulae:

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

                                    Metal            Oxygen

              Mass               a                       b

              Mole              a / Mr              b / Mr

                                     = x                   = y

              Ratio                x          :           y

Step 3 – Represent the Ratio into the Form ‘ xO ‘ E.g, MgO

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Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.