A-LevelChemistryAnalytical techniques9701/33 · Feb/Mar 2022 · Q313 Marks

FA 7 and FA 8 are solutions containing a total of three cations and two anions. Two of the cations and both of the anions are listed in the Qualitative analysis notes.

(a)(i)Carry out the following tests and record your observations. Use a fresh 1cm depth of solution in a test-tube for each test. test | observations FA 7 | FA 8 Test 1 Add a 1 cm depth of dilute nitric or hydrochloric acid and allow to stand for 2 minutes, then add a few drops of aqueous barium nitrate or aqueous barium chloride. Test 2 Add a few drops of acidified aqueous potassium manganate(VII). Test 3 Add a few drops of aqueous iron(III) chloride and allow to stand for 1 minute.[4]
(a)(ii)From your test results, give the formulae of the anions present in FA 7 and FA 8. If the tests do not allow you to positively identify an anion, write 'unknown'. anion in FA 7 = ...................................... anion in FA 8 = ......................................[2]
(b)(i)Select reagents for tests to identify as many of the cations as possible in FA 7 and FA 8. Carry out your tests and record your reagents, conditions and observations.[4]
(b)(ii)From your test results, give the formulae of as many cations as possible in FA 7 and FA 8. If the tests do not allow you to positively identify a cation, write 'unknown'. FA 7 contains ...................................... FA 8 contains ......................................[2]
(b)(iii)Write an ionic equation for one reaction you observed in (b)(i). Include state symbols.[1]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2022). The mark or mark type is shown in brackets.

[0.5] FA 7 (Test 1 + H+): (slow formation of) white/off-white / cream / pale yellow ppt
[0.5] FA 8 (Test 1 + H+): no change / no (visible) reaction / no ppt
[0.5] FA 7 (Test 1 + Ba2+): no change (provided ppt reported with H+) / ppt remains / insoluble
[0.5] FA 8 (Test 1 + Ba2+): white ppt
[0.5] FA 7 (Test 2 + MnO4-): purple to colourless / KMnO4 decolourises
[0.5] FA 8 (Test 2 + MnO4-): no change / no (visible) reaction / stays purple (Not 'no ppt' alone)
[0.5] FA 7 (Test 3 + FeCl3): purple colour fades / turns colourless (on standing)
[0.5] FA 8 (Test 3 + FeCl3): no change / no (visible) reaction / solution stays yellow
[1] FA 7 = S2O3²⁻
[1] FA 8 = SO4²⁻
[1] Reagents: NaOH and NH3 selected; Condition: both NaOH and NH3 used to excess / at least one NaOH mixture warmed.
[3] Observations for FA 7 and FA 8: - With NaOH: FA 7 - no change/no (visible) reaction/no ppt; FA 8 - white ppt, soluble in excess. - With warming: FA 7 - no change to (red) litmus (ignore no reaction/bubbling); FA 8 - gas/NH3 turns (red) litmus blue. - With NH3: FA 7 - Ignore (no observation required); FA 8 - white ppt, insoluble in excess. (Note: 2 x correct observations = 1 mark, round down)
[1] M1: FA 8 contains Al³⁺ (from fully correct observations) OR FA 8 contains NH4⁺ (from litmus turning blue on warming with NaOH).
[1] M2: FA 7 contains 'unknown' AND both Al³⁺ and NH4⁺ are correctly identified for FA 8.
[1] One of: Al³⁺(aq) + 3OH⁻(aq) → Al(OH)3(s) OR Al(OH)3(s) + OH⁻(aq) → [Al(OH)4]⁻(aq) OR NH4⁺(aq) + OH⁻(aq) → NH3(g) + H2O(l) Allow ecf from (b)(ii) on incorrect cations