A-LevelChemistryAnalytical techniques9701/33 · Feb/Mar 2021 · Q116 Marks

In this experiment you will carry out a titration to identify the Group 1 metal, M, present in a metal hydrogencarbonate, MHCO3. FA 1 is 0.0550 moldm⁻³ sulfuric acid, H2SO4. FA 2 is the metal hydrogencarbonate, MHCO3. bromophenol blue indicator

(a)Method Preparing a solution of FA 2 • Weigh the stoppered container of FA 2. Record the mass in the space below. • Tip all the FA 2 into the beaker. • Reweigh the container with its stopper. Record the mass. • Calculate and record the mass of FA 2 used. • Add approximately 100 cm³ of distilled water to FA 2 in the beaker. • Stir the mixture with a glass rod until all the FA 2 has dissolved. • Transfer this solution into the 250 cm³ volumetric flask. • Wash the beaker with distilled water and transfer the washings to the volumetric flask. • Rinse the glass rod with distilled water and transfer the washings to the volumetric flask. • Make up the solution in the volumetric flask to the mark using distilled water. • Shake the flask thoroughly. • This solution of MHCO3 is FA 3. Label the flask FA 3. Titration • Fill the burette with FA 1. • Pipette 25.0 cm³ of FA 3 into a conical flask. • Add a few drops of bromophenol blue indicator to the conical flask. • 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 below all of your burette readings and the volume of FA 1 added in each accurate titration. I II III IV V VI VII VIII [8][8]
(b)From your accurate titration results, obtain a suitable value for the volume of FA 1 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm³ of FA 3 required .................................... cm³ of FA 1. [1][1]
(c)(i)Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) to the appropriate number of significant figures. [1][1]
(c)(ii)Calculate the number of moles of sulfuric acid present in the volume of FA 1 calculated in (b). moles of H2SO4 = .................................... mol [1][1]
(c)(iii)Complete the equation for the reaction of sulfuric acid and MHCO3. State symbols are not required. .....MHCO3 + .H2SO4 → .M2SO4 + .................................... + .................................... Use your answer to (c)(ii) to deduce the number of moles of MHCO3 used in each titration. moles of MHCO3 = .................................... mol [1][1]
(c)(iv)Use your answer to (c)(iii) and your data on page 2 to calculate the relative formula mass, Mr, of MHCO3. Mr of MHCO3 = .................................... [1][1]
(c)(v)Calculate the relative atomic mass, Ar, of M. Suggest the identity of M. Ar of M = .................................... M is .................................... [1][1]
(d)(i)A student used a pipette that was labelled 25.0 ± 0.06 cm³ to measure FA 3. Show how you calculate the maximum percentage error in the volume of FA 3. [1][1]
(d)(ii)The student suggested that it would have been more accurate to measure the volume of FA 3 with a burette instead of the pipette. State and explain whether you agree with the student. [1][1]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2021). The mark or mark type is shown in brackets.

[1] Headings and data are recorded in the space provided • (mass of) container with FA 2 • (mass of empty) container • (mass of) FA 2 (used) Subtraction for the mass of FA 2 used must be correct Headings must be unambiguous and include either 'mass' or g for each piece of datum. Reject 'weight'.
[1] The following data must be shown: • two burette readings and titre for the rough titration • initial and final burette readings for two (or more) accurate titrations
[1] 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 1 and used/added reject 'difference' or 'total' or 'amount' or 'V' but allow ‘vol' • unit: / cm³ or (cm³) or in cm³ for each heading or cm³ unit given for each volume recorded
[1] All accurate burette readings are recorded to the nearest 0.05 cm³, including 0.00. Reject 50(.00) as an initial burette reading Reject if more than one final burette reading is 50.(00) Reject any burette reading is greater than 50.(00)
[1] The final accurate titre recorded is within 0.10 cm³ of any other accurate titre Ignore any titre labelled 'rough’ Reject if any 'accurate' burette reading is recorded as an integer (apart from an initial 0 cm³)
[1] Candidate calculates the mean correctly: • Candidate must take the average of two (or more) accurate 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 should be quoted to 2 dp, and be rounded to nearest 0.01 cm³
[1] All answers given to (c)(ii) – (c)(v) must be to 3 or 4 sig fig (Minimum 3 answers required to award the mark)
[1] Correctly calculates: no of moles of H2SO4 used = 0.0550 × mean titre/1000 The candidate's mean titre must be used.
[1] Correct equation and correctly uses (ii) • 2MHCO3 + H2SO4 → M2SO4 + 2CO2 + 2H2O Allow multiples and ignore state symbols. AND • no of moles of MHCO3 = 2 × answer (ii)
[1] Correctly uses (iii) Mr = mass of FA 2 used/10 x answer (iii)
[1] Correct use of Mr and appropriate identity of M • Ar = answer (iv) – 61 AND • M identified as Group 1 metal with closest Ar Li 0-14.9; Na 15.0–31.0; K 31.1–62.2; Rb 62.3–109.1; Cs 109.2–250 Reject if the Ar calculated is > 250 or if Ar < 0
[1] Correct expression % error = 0.06/25 × 100 (= 0.24 %) No answer needed but reject incorrect answer. No mark for just 0.24 without some working.
[1] Student is incorrect AND error in burette reading = 2 × 0.05 > 0.06 (or candidate compares the % errors, 0.40 % and 0.24 %) Reject suggestion that error in 1 burette reading is 0.1