In this experiment you will determine the value of x in the formula for hydrated copper(II) sulfate, CuSO4.xH2O. You will first react a solution of Cu2+ ions with excess iodide ions, I-. This reaction produces iodine. 2Cu2+(aq) + 4I-(aq) → 2CuI(s) + I2(aq) The amount of iodine produced can be determined by titrating with thiosulfate ions, S2O32-. I2(aq) + 2S2O32-(aq) → 2I-(aq) + S4O62-(aq) FA 1 is 0.150 moldm-³ sodium thiosulfate, Na2S2O3. FA 2 is dilute sulfuric acid. FA 3 is 1.00 mol dm⁻³ potassium iodide, KI. FA 4 is a solution made by dissolving 32.5g of CuSO4.xH2O in 1.00 dm³ of solution. starch indicator

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

1. [1] two burette readings for the rough titration; titre for rough titration; initial and final burette readings for two (or more) accurate titrations (i.e. 2 × 2 “box”)
2. [1] Appropriate headings and units for accurate titration. and volume FA 1 added recorded for each accurate titre. Headings should match readings. initial / start and (burette) reading / volume (allow vol but not V); final / end and (burette) reading / volume; titre or volume / FA 1 and used/added (but not "difference” or “total” or “change”); unit: / cm³ or (cm³) or in cm³ or cm³ for each entry
3. [1] All accurate burette readings are to the nearest 0.05 cm³. The requirement to record to 0.05 applies to burette readings, including 0.00 cm³ (if this was the initial reading), but it does not apply to the titre. Do not award this mark if: 50(.00) is used as an initial burette reading; more than one final burette reading is 50.(00); any burette reading is greater than 50.(00)
4. [1] The final accurate titre recorded is within 0.10 cm³ of any other accurate titre. Examiner rounds any accurate burette readings to the nearest 0.05 cm³ and then selects the 'best' titres using the hierarchy: two (or more) accurate identical titres, then; two (or more) accurate titres within 0.05 cm³, then; two (or more) accurate titres within 0.10 cm³ etc. These best titres should be used to calculate the mean corrected titre to the nearest 0.01 cm³. Examiner compares candidate's titre value with that of the Supervisor:
5. [1] Award V, VI and VII if δ ≤ 0.30 (cm³)
6. [1] Candidate calculates the mean correctly. Candidate averages two (or more) titres where the total spread is < 0.20 cm³. Working must be shown or ticks must be put next to the two (or more) accurate readings selected. The mean should be quoted to 2 dp, and be rounded to nearest 0.01 cm³. (e.g. 26.666 cm³ must be rounded to 26.67 cm³) Two special cases, where the mean need not be to 2 dp: Allow mean to 3 dp only for 0.025 or 0.075 (e.g. 26.325 cm³); Allow mean to 1 dp, if all accurate burette readings were given to 1 dp and the mean is exactly correct. (e.g. 26.0 and 26.2 = 26.1 is allowed) (e.g. 26.0 and 26.1 = 26.1 is wrong – should be 26.05) Do not award this mark if: The rough titre was used to calculate the mean.; The candidate performed only one accurate titration.; Burette readings were incorrectly subtracted to obtain any of the accurate titre values.; All burette readings (resulting in titre values used in calculation of mean) are integers. Note: the candidate's mean will sometimes be marked correct even if it was different from the mean calculated by the Examiner for the purpose of assessing accuracy.
7. [1] Correctly calculates Number of moles of S2O32- used = 0.150 × (b)/1000; Answer given to 3 or 4 sf
8. [1] Correctly calculates ans(ii) = ans(i); Answer given to 3 or 4 sf
9. [1] Correct use ans(ii) / 0.0250 (or equivalent); Answer given to 3 or 4 sf
10. [1] Correct expression 32.5/ans(iii) – 159.6
11. [1] Correct answer x = nearest integer to [32.5/ans(iii) – 159.6]/18
12. [1] Correct expression Use of 0.1(0) / any accurate titre ×100
13. [1] The volume from the burette has a smaller error / more precise
14. [1] FA 3 is in excess