Core Practical 1.36

Core Practical 1.36: Formula of a Metal Oxide

Revision guide containing method, variables, safety, sample calculations, and model exam answers.

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Scientific Principles & Theory

Scientific Background: The empirical formula is the simplest whole-number ratio of atoms of each element in a compound. This can be determined by finding the mass of the metal and the mass of the oxygen that combines with it.

For magnesium oxide, magnesium metal is heated in air to react with oxygen (combustion):

2Mg(s) + O₂(g) → 2MgO(s)

For copper(II) oxide, the oxide is heated in a stream of methane or hydrogen gas. The gas reduces the metal oxide to pure metal, forming water or carbon dioxide as byproducts:

CuO(s) + H₂(g) → Cu(s) + H₂O(g)

Experimental Variables

Independent Variable

Mass of the starting metal (magnesium) or metal oxide (copper oxide).

Dependent Variable

Mass of the product (magnesium oxide or reduced copper metal).

Control Variables

Purity of the starting material, duration of heating (heating to constant mass).

⚠️ Lab Risk Assessment

Hazard Associated Risk Control Measure
Intense UV light from burning magnesium Damage to eyes Do not look directly at the burning magnesium; view through safety screen or lid gaps only.
Hot crucible / glass tubes Thermal burns Allow all apparatus to cool completely before weighing; use crucible tongs to lift the lid.

Apparatus & Procedure

Required Apparatus

  • Crucible and lid
  • Pipeclay triangle
  • Tripod, Bunsen burner, and heatproof mat
  • Digital balance (0.01 g resolution)
  • Emery paper (to clean oxide layer off magnesium)
  • Magnesium ribbon (approx. 10 cm)
  • Crucible tongs

Step-by-Step Procedure

  1. Clean a 10 cm length of magnesium ribbon with emery paper to remove the dull oxide layer and expose the shiny metal.
  2. Weigh the empty, clean crucible and its lid. Record this mass (m₁).
  3. Coil the magnesium ribbon loosely around a pencil, place it inside the crucible, and weigh the crucible, lid, and magnesium ribbon together. Record this mass (m₂).
  4. Set up the tripod, Bunsen burner, and pipeclay triangle. Place the crucible on the triangle.
  5. Heat the crucible strongly with a roaring Bunsen flame.
  6. Use crucible tongs to lift the lid slightly every 30 seconds. This admits oxygen from the air to sustain the reaction, but prevents white magnesium oxide smoke (which contains product particles) from escaping.
  7. Once the magnesium stops glowing and turns into a white powder, remove the lid completely and heat strongly for another 2 minutes.
  8. Turn off the Bunsen burner. Replace the lid and allow the crucible to cool to room temperature.
  9. Weigh the crucible, lid, and magnesium oxide product. Record this mass (m₃).
  10. Heat the crucible strongly again for 2 minutes, let it cool, and re-weigh. Repeat this until a constant mass is obtained.
Determining Formula of Magnesium Oxide Crucible lid (tilted) Pipeclay triangle

Fig 1. Laboratory experimental setup for Core Practical 1.36.

Sample Data & Calculations

This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:

Measurement Parameter Mass (g)
Mass of empty crucible + lid 32.40
Mass of crucible + lid + magnesium ribbon 32.64
Mass of crucible + lid + magnesium oxide product 32.80

Data Processing & Analysis

  1. Mass of magnesium reacted = 32.64 g - 32.40 g = 0.24 g
  2. Mass of oxygen combined = 32.80 g - 32.64 g = 0.16 g
  3. Moles of Mg = 0.24 g / 24 g/mol = 0.010 mol
  4. Moles of O = 0.16 g / 16 g/mol = 0.010 mol
  5. Molar Ratio (Mg : O) = 0.010 : 0.010 = 1 : 1
  6. Empirical Formula = MgO

Conclusion & Evaluation

Chemical Explanation: Saturated solutions are heavily dependent on temperature. Heating shifts solubility limits, allowing more solute to form coordinate bonds or ion-dipole interactions with solvent molecules. When cooled, the reverse process happens and solute precipitates out.

Experimental Error Analysis

Error Type & Source Effect on Final Result Mitigation Strategy
Systematic Error
Magnesium oxide smoke escaping from crucible		 Product mass is lower than it should be, resulting in an calculated ratio of oxygen that is too low. Ensure the lid is replaced quickly and only lifted by a small gap for a split second during heating.
Systematic Error
Incomplete reaction (black unreacted metal inside)		 The mass of combined oxygen is underestimated, leading to an incorrect empirical ratio. Clean the starting ribbon thoroughly, heat strongly, and repeat heating to constant mass.

Exam Practice

Exam-Style Design Question (6 Marks)

Plan an experiment to determine the empirical formula of magnesium oxide by combustion. Describe the steps you would take, the measurements you would record, and how you would process the data to find the formula.

View Model Answer & Mark Scheme

Model Answer (6/6 Marks):

  1. Cleaning: Rub the magnesium ribbon with emery paper to remove any surface oxide layer, ensuring only pure magnesium reacts.
  2. Initial Weighing: Weigh the empty crucible with its lid. Then place the coiled magnesium ribbon inside and weigh the crucible, lid, and magnesium together.
  3. Heating: Heat the crucible strongly over a Bunsen burner on a pipeclay triangle.
  4. Lid Control: Periodically lift the lid slightly with tongs to let oxygen enter, but close it quickly to prevent white magnesium oxide smoke from escaping.
  5. Final Heating & cooling: Remove the lid completely near the end to ensure complete combustion. Allow to cool and weigh.
  6. Constant Mass: Re-heat, cool, and re-weigh until two consecutive mass readings are identical.
  7. Calculation: Determine mass of Mg and O. Divide each mass by its relative atomic mass (Mg = 24, O = 16) to find moles. Divide by the smallest mole value to find the simplest whole-number ratio.
Examiner Tip:

Always emphasize the reason for lifting the lid (let oxygen in) and the reason for closing it quickly (prevent loss of product smoke).