A student prepares two solutions of hydrochloric acid. Solution A has a concentration of 2.0 mol/dm³ and Solution B has a concentration of 0.5 mol/dm³. Explain what is meant by the term concentration and describe how the student could change Solution A into Solution B without making a fresh solution.

AQA GCSE Chemistry (8462) — 4.3.4 Using concentrations of solutions in mol/dm³ (Chem only) · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Concentration is the amount of solute (in moles) present in a given volume of solution.
To change Solution A into Solution B the student can add water to Solution A.
Adding water increases the total volume while the number of moles of HCl remains unchanged.
The increased volume lowers the molarity, giving the 0.5 mol dm⁻³ concentration of Solution B.

Examiner tips

  • Use the term ‘molarity’ or ‘mol dm⁻³’ when defining concentration.
  • Show that moles of solute stay constant and volume increases to explain the drop in concentration.

Common mistakes

  • Confusing concentration with amount of solute, e.g. saying ‘concentration is the amount of solution’. Adding solute instead of water, i.e. increasing moles rather than diluting.

Mark scheme (4 marks)

  1. Concentration is the amount of substance (in moles) dissolved in a given volume of solution
  2. The student should add more water / solvent to Solution A (dilution)
  3. The number of moles of solute stays the same / is unchanged when water is added
  4. The volume of the solution increases, which decreases the concentration

Key terms in this question

concentration · mol/dm³

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