A student makes a solution of citric acid for a food science experiment. The student uses 0.050 mol of citric acid and dissolves it in 250 cm³ of water. Explain what happens to the concentration of the citric acid solution if the student had instead dissolved the same amount of citric acid in 500 cm³ of water.

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)

The concentration would be lower. The same 0.050 mol of citric acid is now dissolved in a larger volume of solution. The original concentration is 0.050 mol ÷ 0.250 dm³ = 0.20 mol/dm³. If the volume is doubled to 0.500 dm³, the new concentration is 0.050 mol ÷ 0.500 dm³ = 0.10 mol/dm³, i.e. half the original value.

Examiner tips

  • Show the calculation for the original concentration first.
  • Explain that concentration = moles ÷ volume and that doubling the volume halves the concentration.
  • Use the term ‘halved’ to match the mark scheme.
  • Mention the units (mol/dm³).

Common mistakes

  • Using the wrong volume unit (cm³ instead of dm³).
  • Failing to divide the moles by the new volume to show the halving effect.
  • Not showing the calculation for the original concentration.

Mark scheme (4 marks)

  1. The concentration would decrease / be lower / be halved
  2. Because the same amount (number of moles) of solute is dissolved in a larger volume of solution
  3. Original concentration = 0.050 ÷ 0.250 = 0.20 mol/dm³
  4. New concentration = 0.050 ÷ 0.500 = 0.10 mol/dm³

Key terms in this question

concentration · solution

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