1.5.7Principles of Chemistry
Practical: Determine the Formula of a Metal Oxide — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 29 of 50
Part 1 of 5Choosing a method
| Feature | Magnesium oxide | Copper(II) oxide |
|---|---|---|
| Type of change | Oxidation: the metal gains oxygen | Reduction: the oxide loses oxygen |
| Starting material | Magnesium ribbon | Copper(II) oxide powder |
| Apparatus | Crucible with lid, tongs, Bunsen burner, balance | Hard-glass tube, clamp, supply of methane, Bunsen burner, balance |
| Mass change | Increases as oxygen is added | Decreases as oxygen is removed |
| Why this method | Magnesium reacts readily with oxygen in air | Copper is unreactive, so methane can act as a reducing agent and remove its oxygen |
Part 2 of 5Method A: heating magnesium in a crucible
1
Weigh the empty crucible with its lid.
2
Add a coil of clean magnesium ribbon and weigh the crucible, lid and magnesium. Subtract to find the mass of magnesium.
3
Heat the crucible strongly on a pipeclay triangle over a Bunsen burner for several minutes.
4
Lift the lid slightly and often so that air can get in and the magnesium burns completely, but replace it quickly so no white oxide smoke escapes.
5
Keep heating, cooling and reweighing until the mass stops increasing, which shows all the magnesium has reacted.
6
Weigh the cooled crucible, lid and magnesium oxide. Subtract the empty crucible to find the mass of oxide.
Part 3 of 5Method B: reducing copper(II) oxide
1
Weigh an empty boiling tube, add some copper(II) oxide and weigh again to find the mass of oxide.
2
Clamp the tube horizontally and pass a steady stream of methane (natural gas) over the oxide, burning off the excess gas at the end of the tube.
3
Heat the copper(II) oxide strongly with a Bunsen burner while the gas flows over it.
4
Continue until all the black oxide has turned into pink-brown copper, which means all the oxygen has been removed.
5
Let the tube cool while the gas is still flowing, so hot copper is not re-oxidised by air, then weigh the tube and copper. Subtract the tube mass to find the mass of copper.
Part 4 of 5Processing the results
1Mass of metal and oxygen
- Mass of metal and oxygen For magnesium: oxygen = mass of oxide − mass of metal.
- For copper: oxygen = mass of oxide − mass of copper left.
2Divide by Ar
- Divide by Ar Divide the mass of each element by its relative atomic mass to give moles: Mg = 24, Cu = 64, O = 16.
3Simplify the ratio
- Simplify the ratio Divide both mole values by the smaller to reach the simplest whole-number ratio, x : y.
4Write the formula
- Write the formula Write it as MxOy. Example: 0.60 g Mg and 0.40 g O gives 0.025 : 0.025 = 1 : 1, so MgO.
Part 5 of 5Variables, safety and accuracy
Variables
- Variables The independent variable is the metal or oxide used;
- the dependent variable is the change in mass;
- the heating time and use of the same balance are controlled.
Safety
- Safety Wear eye protection and never look directly at burning magnesium, because the bright flame can damage eyes.
- Use tongs for the hot crucible and keep methane away from naked flames other than the burner.
Sources of error
- Sources of error If smoke escapes, the oxide mass is too low and the oxygen is under-estimated.
- If the magnesium does not fully burn, the oxide mass is also too low and the ratio is wrong.
Key terms
| Term | Meaning |
|---|---|
| Empirical formula | The simplest whole-number ratio of atoms of each element in a compound. |
| Oxidation | A reaction in which a substance gains oxygen. |
| Reduction | A reaction in which a substance loses oxygen. |
| Crucible | A small heat-resistant pot used for heating solids to high temperatures. |
| Constant mass | Repeated heating and weighing until the mass stops changing. |
Avoid these mistakes
1
Dividing the masses by each other instead of by the relative atomic masses; the ratio must be in moles, not grams.
2
Forgetting to subtract the crucible or tube mass before doing the calculation.
3
Letting magnesium oxide smoke escape and then not recognising that this causes a low result.
4
Switching off the methane before the copper has cooled, so air re-oxidises the copper and the mass is too high.
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