Changing the concentration of the reactants
4 marks · NormalA student is given a bottle of 2.00 mol/dm³ hydrochloric acid. She prepares diluted solutions for a reaction experiment. The total final volume of each solution will be 25.0 cm³.
Calculate the concentration (in mol/dm³) if she mixes 8.0 cm³ of the original acid with 17.0 cm³ of distilled water.
Calculate the concentration (in mol/dm³) if she instead uses 20.0 cm³ of the original acid and 5.0 cm³ of water, making the total volume up to 25.0 cm³.
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(a) 0.64 mol/dm³
(b) 1.60 mol/dm³
Common mistakes: Wrong volumes in formula; Forgetting to use total volume
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Substitutes values into dilution formula: (8.0 / 25.0) × 2.00. (follow-through)
A1 (a) · 1 mark · Correct answer for part (a): 0.64 mol/dm³ (allow 2 decimal places). (follow-through)
M2 (b) · 1 mark · Uses (20.0 / 25.0) × 2.00 for dilution calculation. (follow-through)
A2 (b) · 1 mark · Correct answer for part (b): 1.60 mol/dm³ (allow 2 decimal places). (follow-through)
Worked solution
Use: concentration = (volume of acid / total volume) × original concentration