A-LevelChemistryAnalytical techniques9701/32 · May/June 2022 · Q312 Marks

FB 6 is the solid impurity found in the bottle of solid used to prepare FB 2. It is a compound of a Group 1 metal and does not contain sulfur. The anion in FB 6 is listed in the Qualitative analysis notes.

(a)Select a reagent, or reagents, and carry out one test on FB 6 to collect more information about the anion present. reagent or reagents apparatus and conditions observations From your observations give the formula of the anion present. If you are unable to identify the anion positively from your test and observations, then write 'unknown'. formula of anion[4]
(b)(i)FB 7 and FB 8 are solutions containing a total of three cations. All of the cations are listed in the Qualitative analysis notes. Carry out the following tests and record your observations. Use a fresh 1 cm depth of solution in a test-tube for each test. Table 3.1 test observations FB 7 FB 8 Test 1 Add a few drops of acidified aqueous potassium manganate(VII). Test 2 Add aqueous ammonia. Test 3 Add aqueous sodium hydroxide, then decant the mixture into a boiling tube and warm gently.[4]
(b)(ii)Using your observations in (b)(i), identify the cations present in FB 7 and FB 8. Write the formula of each cation identified. If the tests do not allow you to positively identify the cations, write 'unknown'. cation or cations in FB 7 cation or cations in FB 8[2]
(b)(iii)Construct the ionic equation for one reaction observed on addition of aqueous ammonia in Test 2. Include state symbols.[1]
(b)(iv)Deduce the type of reaction which occurs when acidified aqueous potassium manganate(VII) is added to FB 7 in Test 1 in (b)(i).[1]

Mark scheme

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

FB 6 is KI(s); FB 7 is MnCl2 + NH4Cl(aq); FB 8 is FeSO4
Suitable reagents for anion One of: If testing for carbonate: I named (dilute) acid (and limewater) II description of testing gas with limewater III no effervescence / no fizzing / no change IV unknown (provided test carried out and no contradictory result) OR If testing for halide: I (aqueous) AgNO3 and NH3 II use a solution of FB 6 III (pale) yellow ppt, insoluble in NH3 IV I- (must be from (pale) yellow ppt) OR If testing for nitrate / nitrite : I sodium hydroxide and Al II warm in a boiling tube III gas does not turn litmus blue / red litmus stays red IV unknown (provided test carried out and no contradictory result)
FB 7: turns brown OR brown solution OR (dark) brown ppt *
FB 8: purple (solution) / KMnO4/MnO4- AND decolourised / turns yellow / turns colourless *
FB 7: off-white / pale brown ppt AND insoluble in excess OR darkens / turns dark(er) brown *
FB 8: green ppt AND insoluble in excess OR turns brown/red-brown / rust (on standing) *
FB 7: off-white / pale brown ppt AND insoluble in excess OR darkens / turns dark(er) brown *
FB 8: green ppt AND insoluble in excess OR turns brown/red-brown / rust (on standing) *
FB 7: gas/NH3 turns (damp red) litmus blue *
FB 8: no (visible) reaction / no change *
2* = 1 mark
[2] FB 7: Mn2+ and NH4+ FB 8: Fe2+ 3 cations correct = 2 marks 2 cations correct = 1 mark
[1] One of: Mn2+(aq) + 2OH-(aq) → Mn(OH)2(s) Fe2+(aq) + 2OH¯(aq) → Fe(OH)2(s)
[1] Redox / oxidation AND reduction (of manganate(VII))