A-LevelChemistryAtoms, molecules and stoichiometry9701/31 · Oct/Nov 2019 · Q216 Marks

In this experiment you will determine the concentration of FA 2 by titration using aqueous sodium hydroxide. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) FA 2 is hydrochloric acid, HCl. FA 3 is 0.100 mol dm⁻³ sodium hydroxide, NaOH. methyl orange indicator

(a)Method Dilution of FA 2 • Fill the burette with FA 2. • Run between 40.00 and 45.00 cm³ from the burette into the 250 cm³ volumetric flask. • Record the volume used. • Make the solution up to the 250 cm³ mark by adding distilled water. • Shake the flask thoroughly to ensure mixing. • Label this solution of hydrochloric acid FA 4. volume of FA 2 used = .............................................. cm³ Titration • Rinse the burette with distilled water and then with a little FA 4. • Fill the burette with FA 4. • Pipette 25.0 cm³ of FA 3 into a conical flask. • Add several drops of methyl orange indicator. • Perform a rough titration and record your burette readings. 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 all of your burette readings and the volume of FA 4 added in each accurate titration. [8][8]
(b)From your accurate titration results, obtain a value for the volume of FA 4 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm³ of FA 3 required .............................................. cm³ of FA 4. [1][1]
(c)(i)Give your answers to (ii), (iii) and (iv) to the appropriate number of significant figures. [1][1]
(c)(ii)Calculate the number of moles of hydrochloric acid that reacted with 25.0 cm³ of FA 3. moles of HCl = .............................................. mol [1][1]
(c)(iii)Calculate the concentration of hydrochloric acid in FA 4. concentration of HCl in FA 4 = .............................................. mol dm⁻³ [1][1]
(c)(iv)Calculate the concentration of hydrochloric acid in FA 2. concentration of HCl in FA 2 = .............................................. mol dm⁻³ [1][1]
(d)Calculate the maximum percentage error in the volume of FA 2 you added to the volumetric flask. maximum percentage error = .............................................. % [1][1]
(e)In Question 1 and Question 2 you have determined the concentration of FA 2 by two different methods. Each method used has possible sources of error, for example in Question 1 the largest source of error is escape of gas. Apart from this error, state and explain a source of error for each method. Question 1 Question 2 [2][2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

[1] Uses a volume between 40.00 and 45.00 cm³ and answer to at least 1 dp
[1] II The following data must be shown • burette readings and titre for rough titration • 2 × 2 'box' showing both accurate burette readings
[1] III Headings and units correct for accurate titration table and headings match readings. • Initial / start (burette) and reading / volume + unit • Final / end (burette) and reading / volume + unit • titre or volume / FA 4 and used / added (not 'difference' amount or 'total') + unit
[1] IV All accurate burette readings to 0.05 cm³
[1] ✓ The final accurate titre recorded is within 0.10 cm³ of any other accurate titre.
[1] Award VI if 20 < δ ≤ 30 cm³
[1] Award VII if 10 < δ < 20 cm³
[1] Award VIII if δ < 10 cm³
[1] Candidate must average two (or more) titres that are all within 0.20 cm³. Working must be shown or ticks must be put next to the two (or more) accurate titres selected.
[1] Answers for (ii), (iii) and (iv) given to 3–4 sf. Minimum three answers displayed.
[1] Correctly calculates 2.50 × 10⁻³
[1] Correct use of ans (c)(ii) × 1000 / ans (b)
[1] Correct expression: ans (c)(iii) × 250 / vol used from (a)
[1] Correctly calculates 0.10 / vol used in (a) × 100.
[1] Question 1 • measuring cylinder greater error than burette / pipette • molar gas volume of 24 dm³ may not be valid / temperature of the lab may not be known • too much gas for the measuring cylinder (check that vol > 250 cm³) • use gas syringe (if volume < 100 cm³)
[1] Question 2 • dilution introduces extra stage / greater cumulative error • methyl orange end-point can be difficult to see / colour change gradual / difficult to see