A student makes a solution by dissolving potassium nitrate (KNO₃) in water. The concentration of the solution is 0.5 mol/dm³. Explain what is meant by a concentration of 0.5 mol/dm³ and describe how the concentration of this solution would change if more water were added to it.

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 substance in a given volume of solution.
0.5 mol/dm³ means there are 0.5 moles of KNO₃ in every 1 dm³ (litre) of solution.
If more water is added the volume of the solution increases.
Because the same 0.5 moles of KNO₃ are now dispersed in a larger volume, the concentration decreases.

Examiner tips

  • Use the definition of concentration first. State the numerical meaning of 0.5 mol/dm³. Explain the effect of adding water on volume and concentration. Keep the answer concise and use correct units.

Common mistakes

  • Confusing mol/dm³ with mol/L. Saying the concentration increases instead of decreases. Omitting the definition of concentration.

Mark scheme (4 marks)

  1. Concentration is the amount of substance in a certain volume of solution
  2. 0.5 mol/dm³ means there are 0.5 moles of KNO₃ in every 1 dm³ (litre) of solution
  3. Adding more water increases the volume of the solution
  4. The concentration decreases because the same number of moles of KNO₃ is now in a larger volume

Key terms in this question

concentration · mol/dm³ · solution

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