A-LevelChemistryAtoms, molecules and stoichiometry9701/32 · May/June 2023 · Q216 Marks

In this experiment you will determine the relative atomic mass, A, of another metal, X, by a titration method using the metal carbonate, X2CO3. FB 2 is 0.0460 moldm¯³ hydrochloric acid, HCl. FB 3 is the metal carbonate, X2CO3. FB 4 is methyl orange indicator.

(a)Method Preparing a solution of FB 3 • Weigh the stoppered container of FB 3. Record the mass in the space below. • Tip all of the FB 3 into the 250 cm³ beaker. • Reweigh the container with its stopper. Record the mass. • Calculate and record the mass of FB 3 used. • Add approximately 100cm³ of distilled water to FB 3 in the beaker. • Stir the mixture with a glass rod until all the FB 3 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 X2CO3 is FB 5. Label the flask FB 5. Titration • Fill the burette with FB 2. • Pipette 25.0cm³ of FB 5 into a conical flask. • Add several drops of FB 4 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 FB 2 added in each accurate titration. I II III IV V VI VII VIII[8]
(b)From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtain the mean value. 25.0cm³ of FB 5 required ................................. cm³ of FB 2. [1][1]
(c)(i)Give your answers to (c)(ii), (c)(iv), (c)(v) and (c)(vi) to an appropriate number of significant figures.[1]
(c)(ii)Calculate the amount, in mol, of hydrochloric acid present in the volume of FB 2 in (b).[1]
(c)(iii)Give the ionic equation for the reaction of hydrochloric acid with the metal carbonate during the titration. Include state symbols.[1]
(c)(iv)Calculate the concentration of X2CO3, in moldm⁻³, in FB 5.[1]
(c)(v)Calculate the relative formula mass, Mr, of X2CO3.[1]
(c)(vi)Calculate the relative atomic mass of X.[1]
(c)(vii)Identify X.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

[8] I Data from weighings of FB 3 recorded in space provided • three masses recorded, with unambiguous headings and unit • subtraction correct (for mass of FB 3) • mass of FB 3 used is between 0.80 and 1.80 g (from weighings)
II All the following data are recorded • two burette readings AND titre for the rough titration • initial and final burette readings for two (or more) accurate titrations
III Appropriate headings and units in the accurate titration table and titre values recorded for accurate titrations • initial / start AND (burette) reading / volume • final / end AND (burette) reading / volume • titre or volume / FB 2 AND used / added • unit: / cm³ or (cm³) or “in cm³” (for each heading) or cm³ unit given for each volume recorded
IV All accurate burette readings are recorded to the nearest 0.05 cm³
V The final accurate titre recorded is within 0.10 cm³ of any other accurate titre
Accuracy marks The Examiner should round all burette readings to the nearest 0.05 cm³. Check and correct subtractions. Then select the “best” titres using the hierarchy: • two (or more) accurate identical titres (ignoring any that are labelled “rough”), 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 titre, expressed to nearest 0.01 cm³. Calculate the Supervisor's ratio (mean titre / mass of FB 3) to two decimal places. Calculate the candidate's ratio (mean titre / mass of FB 3) to two decimal places. Calculate the difference (δ) between the candidate's ratio and the supervisor's ratio. Award VI if δ < 0.80 Award VII if δ < 0.50 Award VIII if δ ≤ 0.30 (cm³g-1)
[1] Correctly calculates 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 shown 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. 26.625 cm³ must be rounded to 26.63 cm³)
[1] All answers to parts (c)(ii), (c)(iv), (c)(v) and (c)(vi) given to 3 or 4 significant figures.
[1] Correct calculation of number of moles of HCl number of moles = vol in (b) x 0.046 / 1000
[1] Ionic equation correct (including state symbols) CO3²⁻(aq) + 2H⁺(aq) → CO2(aq) + H2O(l)
[1] Correct use of (b), (c)(ii) and (c)(iii) to calculate concentration of X2CO3 [X2CO3] = 0.5 × (c)(ii) × 1000 / 25 mol dm⁻³
[1] Calculation of Mr of FB 3 Mr of X2CO3 = 4 × mass FB 3 used / (c)(iv)
[1] Calculation of Ar from Mr Ar of X = (Mr - 60)/2
[1] Appropriate name / symbol of X X must be in Group 1 and have nearest Ar to value calculated in (c)(vi)