A student dissolves 4.0 g of sodium hydroxide (NaOH) in water to make 500 cm³ of solution. Describe how the concentration of this solution would change if the student had instead dissolved the same 4.0 g of sodium hydroxide in only 250 cm³ of water. Explain why this change occurs.

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).

Concentration is defined as the amount of substance in a certain volume of solution. The relative formula mass of NaOH is 40.

Model answer (4 marks)

The concentration would increase.
The concentration would double.
The same 4.0 g of NaOH (0.10 mol) is dissolved in a smaller volume.
There are twice as many moles of NaOH per dm³, so the concentration is twice as great.

Examiner tips

  • Use the word ‘increase’ or ‘be higher’ for the first point. State ‘double’ or ‘twice as great’ for the second. Mention the unchanged mass/moles of solute. Explain that moles per unit volume are higher.”]
  • common_mistakes
  • :
  • Writing ‘decrease’ instead of ‘increase’. Forgetting to state that the mass of NaOH is unchanged. Using ‘half the concentration’ instead of ‘double’.”

Mark scheme (4 marks)

  1. The concentration would increase / be greater / be higher
  2. The concentration would double / be twice as great
  3. The same amount (mass/moles) of solute is dissolved in a smaller volume of solvent/solution
  4. There are more particles/moles of solute per dm³ (unit volume)

Key terms in this question

concentration

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