Iron reacts with sulfur to form iron sulfide. The equation for this reaction is: Fe + S → FeS. Explain why the mass of iron sulfide produced is greater than the mass of iron used, even though no atoms are lost or made during the reaction. Use the idea of relative formula mass in your answer.

AQA GCSE Chemistry (8462) — 4.3.2 Use of amount of substance in relation to masses of pure substances · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A student heats 5.6 g of iron with an excess of sulfur. After the reaction, the student notices that the mass of the solid product is greater than 5.6 g.

Model answer (4 marks)

The law of conservation of mass states that no atoms are lost or made during a chemical reaction. In the reaction Fe + S → FeS, both iron and sulfur atoms are incorporated into the product. The mass of iron sulfide therefore contains the mass of the iron used plus the mass of the sulfur that reacted. The relative formula mass of FeS is 88, which is greater than the relative atomic mass of Fe alone (56). Consequently, the mass of the iron sulfide produced is greater than the mass of the iron alone because it also includes the mass of the sulfur that reacted.

Examiner tips

  • Use the word ‘conservation of mass’ early to show understanding of the law.
  • Explain that both reactants contribute to the product’s mass.
  • Mention the relative formula mass of FeS (88) versus Fe (56).
  • Show the logical link that the product’s mass is the sum of both reactants’ masses.

Common mistakes

  • Failing to mention that sulfur also contributes to the product’s mass.
  • Using the wrong relative formula mass (e.g., 56 instead of 88 for FeS).
  • Not explicitly linking the conservation of mass to the increase in mass.

Mark scheme (4 marks)

  1. The law of conservation of mass states that no atoms are lost or made during a chemical reaction
  2. Both iron and sulfur atoms are incorporated into the product, so the mass of iron sulfide includes the mass of both reactants
  3. The relative formula mass of FeS (88) is greater than the relative atomic mass of Fe alone (56)
  4. Therefore the product has a greater mass than the iron alone because it also includes the mass of the sulfur that reacted

Key terms in this question

relative formula mass

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