The questions are on the titration test. Take it under real conditions before reading the mark scheme.
Quick answer: A titration finds an unknown concentration by measuring the exact volume of one solution that reacts with a known volume of another. This mark scheme gives the model answers and the marks for the 8 questions of the Chemistry 9-10 test on titration, from apparatus to mass concentration and rough titre, with the equivalent answers to accept.
Model answers and mark schemes for the test. Award method marks even if an earlier error is carried forward, and credit correct alternative explanations about technique.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE C4 RP
Question 1 Apparatus (2 marks)
- A pipette.
- It makes the colour change at the end point easier to see.
Marking: 1 mark each.
Question 2 Reading a burette (2 marks)
- The meniscus is at 23.30 cm³.
- 23.30 − 0.10 = 23.20 cm³.
Marking: 1 mark each; allow error carried forward in part b.
Question 3 The mean titre (2 marks)
- The three accurate titres, because they are within 0.10 cm³ (concordant); the rough titre is not used.
- (21.30 + 21.20 + 21.25) ÷ 3 = 21.25 cm³.
Marking: 1 mark each.
Question 4 Potassium hydroxide (3 marks)
- 0.0800 × 0.01875 = 0.001500 mol.
- Ratio 1 : 1; c = 0.001500 ÷ 0.0250 = 0.0600 mol/dm³.
Marking: 1 mark for moles; 1 mark for the ratio; 1 mark for the concentration.
Question 5 Sulfuric acid (4 marks)
- 0.100 × 0.02800 = 0.00280 mol.
- Ratio 1 : 2, so 0.00140 mol.
- 0.00140 ÷ 0.0250 = 0.0560 mol/dm³.
- 0.0560 × 98 = 5.49 g/dm³.
Marking: 1 mark each; allow error carried forward.
Question 6 Choosing an indicator (3 marks)
- Methyl orange.
- The end point is slightly acidic (about pH 5), but phenolphthalein changes colour only in alkaline solution, so it would change before the end point and give a wrong titre.
Marking: 1 mark for the indicator; 1 mark for the end-point pH; 1 mark for the consequence.
Question 7 Effect of an error (2 marks)
- The acid is diluted, so a larger volume is needed to reach the end point.
- The calculated concentration is too high.
Marking: 1 mark each.
Question 8 Mass concentration and rough titre (2 marks)
- M = 56; 0.0600 × 56 = 3.36 g/dm³.
- It shows approximately where the end point is, so the accurate runs can slow down at the right moment.
Marking: 1 mark each.
