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
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
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
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.
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