Propane (C₃H₈) is a hydrocarbon used as a fuel in camping stoves. When propane burns completely in oxygen, the products are carbon dioxide and water. The balanced equation for this reaction is: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Explain why the total mass of carbon dioxide and water produced is always equal to the total mass of propane and oxygen that reacted.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Propane (C₃H₈) is a hydrocarbon fuel. When it burns completely in oxygen, carbon dioxide and water are the only products. The balanced equation for this reaction is: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Model answer (5 marks)
The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction.
In the combustion of propane, no atoms are lost or made – the same atoms that are present in the reactants are simply rearranged to form the products.
The balanced equation shows that the numbers of atoms of each element are equal on both sides: 3 C atoms on each side, 8 H atoms on each side and 10 O atoms on each side.
Because the total number of atoms of each element is unchanged, the total mass of the products (CO₂ + H₂O) must equal the total mass of the reactants (C₃H₈ + O₂).
In the combustion of propane, no atoms are lost or made – the same atoms that are present in the reactants are simply rearranged to form the products.
The balanced equation shows that the numbers of atoms of each element are equal on both sides: 3 C atoms on each side, 8 H atoms on each side and 10 O atoms on each side.
Because the total number of atoms of each element is unchanged, the total mass of the products (CO₂ + H₂O) must equal the total mass of the reactants (C₃H₈ + O₂).
Examiner tips
- Use the phrase "law of conservation of mass" first. Show the equality of atoms from the balanced equation. Link the atom balance directly to mass equality. Keep the answer concise and to the point.
Common mistakes
- Forgetting to mention the law of conservation of mass. Failing to show that the atom counts are equal on both sides. Using vague wording like "mass is conserved" without linking to the balanced equation.
Mark scheme (5 marks)
- States the law of conservation of mass
- No atoms are lost or made / created or destroyed during a chemical reaction
- The same atoms are present on both sides of the equation / in the reactants and products
- Therefore the mass of the products equals the mass of the reactants
- Links to the balanced equation: the balancing numbers show atoms of each element are equal on both sides, e.g. 3 carbon atoms on each side / 8 hydrogen atoms on each side / 10 oxygen atoms on each side
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Hydrocarbons questions
- Crude oil is separated into fractions by fractional distillation. Each fraction …
- Ethene (C₂H₄) is a hydrocarbon. When ethene burns completely in oxygen, carbon d…
- Crude oil is a mixture of hydrocarbons. When a hydrocarbon burns in a limited su…
- Methane is a hydrocarbon that is the main component of natural gas. When methane…