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New
Year 10
Edexcel
Higher

Deducing an empirical formula experimentally: using moles

I can describe and carry out an experiment to determine the empirical formula of a simple compound.

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New
New
Year 10
Edexcel
Higher

Deducing an empirical formula experimentally: using moles

I can describe and carry out an experiment to determine the empirical formula of a simple compound.

Download all resources
Share activities with pupils

Lesson details

Key learning points

  1. Small masses of a substance, including magnesium, can be easily heated in a crucible.
  2. The crucible needs to be opened periodically to allow air (i.e. oxygen) to enter the apparatus.
  3. Lifting a crucible lid too often or for too long may lead to loss of product (i.e. lower final mass recorded).
  4. Using practical data and conservation of mass, chemists can calculate unknown reactant masses.
  5. Reactant masses can determine the empirical formula of a substance, including the ionic molar ratio of a metal & oxygen.

Common misconception

Pupils think that simply ensuring a fair test will improve data collecting in a practical.

Evaluating a practical is an often overlooked skill. Challenge pupils to focus on the equipment chosen and/or the method followed to consider how better data might be collected.

Keywords

  • Empirical formula - A substance's empirical formula shows the simplest whole number ratio of atoms of each element in a compound.

  • Evaluate - To evaluate something is to make a judgement about it with respect to a particular purpose.

  • Mole - A mole of something is 6.02 × 10²³ of it. The mass of a mole of a substance is its relative mass expressed in grams.

  • Relative atomic mass - Relative atomic mass is the mean mass of an atom relative to 1/12th the mass of an atom of carbon–12.

Consider providing sample data (with a method) and challenge pupils to suggest why the data may not be very good/could be improved.
Teacher tip

Equipment

balance (ideally 2 d.p.) magnesium strips (~3 cm) crucible and li. Bunsen burner tripod + clay triangle heatproof mat tongs

Content guidance

  • Risk assessment required - equipment

Supervision

Adult supervision required

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.
What does an empirical formula represent?
The exact number of atoms in a molecule.
Correct answer: The simplest whole number ratio of atoms in a compound.
The total mass of a compound.
The physical properties of a compound.
Q2.
Which empirical formula indicates a 1 : 1 ratio of metal to nonmetal ions?
CaCl₂
Correct answer: NaBr
MgO₂
Al₂O₃
Q3.
The empirical formula demonstrates the simplest whole number ratio of elements, which aligns with the stoichiometric ratios in chemical equations.
Correct Answer: balanced
Q4.
The relative atomic mass of elements is crucial in determining the empirical formula because it ...
identifies isotopes.
Correct answer: helps calculate the molar ratio of elements.
determines chemical reactivity.
indicates physical state changes.
Q5.
If 12 g of carbon react with 32 g of oxygen, what is the empirical formula of the resulting compound?
CO
Correct answer: CO₂
C₂O
C₃O₂
Q6.
A compound contains 32.7% sulfur, 2.0% hydrogen, and 65.3% oxygen combine to form a compound. Determine the empirical formula.
Correct answer: SH₂O₄
SH₄O₂
S₂H₄O
S₄H₂O

Exit quiz

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6 Questions

Q1.
24 g of magnesium reacted with oxygen, producing 40 g of magnesium oxide. Calculate the mass of oxygen that reacted.
Correct Answer: 16 g, 16, 16 grams, 16g
Q2.
Why is it important to periodically open the crucible during the heating of magnesium to produce magnesium oxide?
To let the magnesium cool down.
Correct answer: To allow oxygen to react with the magnesium.
To prevent the magnesium from reacting.
To observe the magnesium's colour change.
Q3.
The combined mass of magnesium and oxygen that reacted is equal to the mass of magnesium oxide produced. This demonstrates the of mass, by accounting for all reactant and product masses.
Correct Answer: conservation
Q4.
Lifting the crucible lid too often, when heating magnesium to form magnesium oxide, can lead to which of the following experimental errors?
A higher final mass.
Correct answer: A lower final mass.
The same final mass.
A more accurate empirical formula.
Q5.
Given that 48 g of magnesium reacted with oxygen and produced 80 g of magnesium oxide, calculate the mass of oxygen that reacted and deduce the empirical formula of magnesium oxide.
Correct answer: MgO
Mg₂O₃
MgO₂
2Mg2O
Q6.
In an experiment, 5.4 g of aluminium reacts with 4.8 g of oxygen to form aluminium oxide. What is the simplest whole number ratio of aluminium to oxygen (Al : O)?
Correct Answer: 2 : 3, 2:3, (2:3), (2 : 3)