HA is an organic acid where A¯ is the anion. You will determine the relative formula mass, M., of HA by titration with sodium hydroxide of known concentration and so identify the anion, A¯. The equation for the reaction is shown. HA(aq) + NaOH(aq) → NaA(aq) + H2O(l) FB 1 is a solution of organic acid, HA, containing 12.60g dm-3. FB 2 is 0.100 mol dm⁻³ sodium hydroxide, NaOH. thymol blue indicator

Mark scheme

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

1. [1] I Initial and final burette readings and volume added recorded for rough titre and accurate titre details tabulated. [minimum 2 × 2 ‘boxes' with relevant information]
2. [1] II Initial and final burette readings recorded and volume of FB 2 added recorded for each accurate titration. Headings and units correct for accurate titrations Headings: initial / final (burette) reading / volume or reading / volume at start / finish and volume / FB 2 added/used or titre and Units: (cm³) or /cm³ or in cm³ [or cm³ by every entry]
3. [1] III All accurate burette readings are recorded to the nearest 0.05 cm³ 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] IV The final accurate titre recorded is within 0.10 cm³ of any other accurate titre.
5. [3] V, VI and VII Award V, VI and VII for δ < 0.20 cm³ Award V and VI for 0.20 cm³ < δ < 0.30 cm³ Award V for 0.30 cm³ < δ ≤ 0.50 cm³
6. [1] Check mean titre is correctly calculated from clearly selected values (ticks or working). Candidate must average 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 normally be quoted to 2 dp rounded to the nearest 0.01. [e.g. 26.667 must be rounded to 26.67] Two special cases where the mean may not be to 2 dp: allow mean to 3 dp only for 0.025 or 0.075 e.g. 26.325; 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 correct but 26.0 and 26.1 = 26.1 is incorrect.] Do not award this mark if: the rough titre was used to calculate the mean; candidate carried out only 1 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 as correct even if it is different from the mean calculated by the examiner for the purpose of assessing accuracy.
7. [1] Correctly calculates 0.100 x 25 / 1000 = 2.5(0) × 10⁻³
8. [1] Correctly calculates 0.0025 x 1000 / (b) to 3 or 4 sf
9. [1] Correct expression 12.6 + (ii)
10. [1] Anion in FB 1 = CHCl₂COO⁻ (allow ecf: for candidate's answer to (iii)) CH₃COO⁻: ≤ 77 CH₂ClCOO⁻: 77.5 – 111.5 CHCl₂COO⁻: 112 - 146 CCl₃COO⁻ : ≥ 146.5
11. [1] Conc NaOH lower = > titre smaller = > smaller Mr
12. [1] No effect on identification unless closer to smaller mass acid or (different Mr may lead to the) identification of a different acid with matching / close to Mr