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A-LevelChemistryAnalytical techniques9701/31 · May/June 2025 · Q218 Marks

You will determine the percentage purity of another sample of calcium carbonate, CaCO3, by titration. The experiment involves three steps. step 1 A known mass of the same impure calcium carbonate, FA 1, is reacted with an excess of hydrochloric acid to form FA 3. This step has been done for you. step 2 You will dilute the products of step 1 to a known volume. step 3 You will carry out a titration to find out how much acid remains after the reaction in step 1. FA 3 has been prepared by reacting 19.0g of FA 1 with 250cm³ of 2.00moldm⁻³ hydrochloric acid, HCl. FA 5 is 0.0900 moldm⁻³ sodium hydroxide, NaOH. FA 6 is bromophenol blue indicator.

  1. (a)Method step 2 • Pipette 25.0 cm³ of FA 3 into the 250 cm³ volumetric flask. • Make the solution up to 250 cm³ with distilled water. • Thoroughly mix the contents of the volumetric flask. This solution is FA 4. step 3 • Fill the burette with FA 5. • Rinse the pipette with distilled water and then with FA 4. • Pipette 25.0 cm³ of FA 4 into a conical flask. • Add several drops of FA 6. • Perform a rough titration and record your burette readings in the space below. The rough titre is ...................................... cm³. • Carry out as many accurate titrations as you think necessary to obtain consistent results. • Make sure any recorded results show the precision of your practical work. • Record, in a suitable form in the space below, all your burette readings and the volume of FA 5 added in each accurate titration. Results[7]
  2. (b)From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtain the mean value. 25.0 cm³ of FA 4 required ...................................... cm³ of FA 5.[1]
  3. (c)(i)Calculations Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) to an appropriate number of significant figures.[1]
  4. (c)(ii)Calculate the amount, in mol, of sodium hydroxide in the volume of FA 5 in (b). amount of NaOH = ...................................... mol[1]
  5. (c)(iii)Use your answer to (c)(ii) to calculate the amount, in mol, of hydrochloric acid in 250 cm³ of FA 4. amount of HCl in 250 cm³ of FA 4 = ...................................... mol Hence, calculate the amount, in mol, of hydrochloric acid present in 250 cm³ of FA 3. amount of HCl in 250 cm³ of FA 3 = ...................................... mol[1]
  6. (c)(iv)Calculate the amount, in mol, of hydrochloric acid used to prepare FA 3. amount of HCl = ...................................... mol[1]
  7. (c)(v)Use your answers to (c)(iii) and (c)(iv) to calculate the amount, in mol, of hydrochloric acid that reacted with the FA 1 in step 1. amount of HCl that reacted with FA 1 = ...................................... mol[1]
  8. (c)(vi)Use your answer to (c)(v) to calculate the amount, in mol, of calcium carbonate present in FA 1. CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g) amount of CaCO3 in FA 1 = ...................................... mol[1]
  9. (c)(vii)Use your answer to (c)(vi) and the mass of FA 1 used to calculate the percentage purity of calcium carbonate. Show your working. purity of calcium carbonate = ...................................... %[1]
  10. (d)(i)The percentage purity of calcium carbonate in FA 1 has been determined by two different methods, gas collection and titration. The gas collection method used in Question 1 is less accurate. Suggest two reasons why the gas collection method is less accurate.[2]
  11. (d)(ii)Describe a change to the gas collection method that would improve the accuracy of the percentage purity determined in Question 1.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

  1. [7] I The following data must be shown: • two burette readings AND titre for rough titration • initial and final burette readings for two (or more) accurate titrations
  2. II Titre values recorded for accurate titrations AND correct headings and units in the accurate titration table • initial / start AND (burette) reading / volume • final / end AND (burette) reading / volume • titre OR volume / FA 5 AND used / added unit: / cm³ OR (cm³) OR in cm³ (for each heading) OR cm³ unit given for each volume recorded
  3. III All accurate burette readings are to nearest 0.05 cm³
  4. IV The final accurate titre recorded must be within 0.10 cm³ of any other accurate titre.
  5. Accuracy (Q) marks: marks V, VI and VII will be determined by comparison of candidate and supervisor Round burette readings to the nearest 0.05 cm³. Check and correct titre subtractions where necessary. Select the best mean titre, using the following hierarchy: • 2 identical accurate titres (ignoring any that are labelled 'rough'), then • accurate titres within 0.05 cm³, then • accurate titres within 0.10 cm³, etc.
  6. V, VI, VII Award V if δ ≤ 0.80 cm³ Award VI if δ < 0.50 cm³ Award VII if δ < 0.30 cm³
  7. [1] Correct calculation of the mean titre • candidate must take the average of two (or more) titres that are within a total spread of not more than 0.20 cm³. • working/ explanation must be shown OR ticks must be put next to the two(or more) accurate readings selected. • the mean must be quoted to 2 decimal place and be rounded to the nearest 0.01 cm³, e.g., 26.675 must be rounded to 26.68.
  8. [1] Significant figures All quoted answers in (c)(ii)-(c)(v) are expressed to 3 or 4 sf.
  9. [1] Correctly calculates amount of NaOH = 0.09 × (vol (b)/1000) mol
  10. [1] Correctly calculates amount of HCl in 250 cm³ FA 4 = (c)(ii) × 10 mol AND amount of HCl in 250 cm³ FA 3 = [(c)(ii) × 10] × 10 mol
  11. [1] Correctly calculates amount of HCl used to prepare FA 3 = (250/1000) × 2 = 0.500 mol
  12. [1] Correctly uses (c)(iv) – (c)(iii)
  13. [1] Correctly uses amount of CaCO3 present = (c)(v)/2 mol
  14. [1] Correctly uses % purity = [(c)(vi) × 100.1 / 19.0] × 100
  15. [2] Any two of: some gas/carbon dioxide dissolves in the water (in trough / measuring cylinder) OR some gas escapes before the bung is replaced on the conical flask OR the impurity may react to produce gas
  16. [1] Any one of: • description of both CaCO3 and acid being separated within the stoppered flask until reaction starts, e.g. use of small container for CaCO3 which can be tipped over, package of CaCO3 held by a thread, etc. OR saturate the water with CO2 prior to experiment / use of warmer water OR use of gas syringe

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About this question

Topic
Analytical techniques, A-Level Chemistry (syllabus 9701). Structured question, 18 marks.
Source
A-Level Chemistry, May/June 2025 Paper 3 Variant 1, question 2.

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