Magnesium reacts with hydrochloric acid according to the equation: Mg + 2HCl → MgCl₂ + H₂. Explain why the law of conservation of mass means that this equation must be balanced, and describe what the balancing numbers in this equation tell us about the amounts, in moles, of each substance involved.

Eduqas A-Level Chemistry — 3.6 Amines and polymers (A-Level only) · Explain · 5 marks · View as Markdown

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

Magnesium is a reactive metal that fizzes vigorously when added to hydrochloric acid, producing a colourless gas.

Model answer (5 marks)

The law of conservation of mass states that no atoms are lost or made during a chemical reaction, so the mass of the products must equal the mass of the reactants. Consequently the equation must be balanced.

The coefficients give the mole ratios:
1. 1 mole of Mg reacts with 2 moles of HCl.
2. 1 mole of MgCl₂ is produced.
3. 1 mole of H₂ gas is produced.

Examiner tips

  • Show the conservation of mass first, then state the mole ratios from the coefficients.
  • Use the word ‘mole’ and give the exact ratios (1:2:1:1).

Common mistakes

  • Writing the equation with unbalanced coefficients.
  • Forgetting to explain that the coefficients represent mole ratios rather than masses.

Mark scheme (5 marks)

  1. The law of conservation of mass states that no atoms are lost or made during a chemical reaction
  2. Therefore the mass of the products equals the mass of the reactants
  3. 1 mole of Mg reacts with 2 moles of HCl
  4. 1 mole of MgCl₂ is produced
  5. 1 mole of H₂ gas is produced

Key terms in this question

conservation of mass · mole · balancing numbers

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