Edexcel A-Level Chemistry Paper 3, November 2021: Question 5

11 marks · Medium difficulty · Practical Techniques and Data Analysis

Identify the ions present in two salts X and Y using qualitative analysis test observations and write ionic equations and formulas for complexes.

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Question

Question text

5 Tests are carried out on aqueous solutions of two salts, X and Y.

(a) X contains one cation and one anion.

The observations for each test are recorded in the table.

(i) Complete the table by writing the names or formulae of the species.

(2)

Test Observation Inference

Test 1 The cation in X is

Add aqueous sodium hydroxide A green precipitate

to an aqueous solution of X forms …

The precipitate turns

brown on the top after

a few minutes

Test 2 The anion in X is

To an aqueous solution of X, add A white precipitate …

dilute hydrochloric acid followed forms

by aqueous barium chloride

(ii) Write the ionic equation for the reaction between the cation in X and aqueous

sodium hydroxide in Test 1. Include state symbols.

(2)

(iii) Give a reason why the green precipitate turns brown on the top after a

few minutes.

(1)

(iv) Give a reason why dilute hydrochloric acid is needed in Test 2.

(1)

(b) Y contains one cation and one anion.

The observations for each test are recorded in the table.

(i) Complete the table by writing the names or formulae of the species.*P67806A01236*

(2)

Test Observation Inference

Test 3 The cation in Y is

Add dilute aqueous ammonia to A pale blue …

an aqueous solution of Y until it precipitate forms

is present in excess The precipitate

dissolves in excess

ammonia to form a

deep blue solution

Test 4 The anion in Y is

To an aqueous solution of Y, add A white precipitate …

dilute nitric acid followed by forms

aqueous silver nitrate

(ii) Give the formula of the complex ion present in the deep blue solution at the

end of Test 3.

(1)

(iii) Give a reagent that could be added to the mixture at the end of Test 4

to confirm the identity of the anion in Y.

Include the observation when this reagent is added.

(2)

(Total for Question 5 = 11 marks)

Mark scheme

Show the mark scheme

Question Answer Additional Guidance Mark

Number

5(a)(i) (2)

• (The cation in X is) Fe2+ / iron(II) / Fe(II) (1) Allow Fe+2

2− (1) Allow sulfate / SO −2

• (The anion in X is) SO4 / sulfate(VI) 4

Do not award sulfite / sulfate(IV)

Number

5(a)(ii) Examples of equation (2)

• species and balancing (1) Fe2+(aq) + 2OH−(aq) → Fe(OH) (s)

(1) [Fe(H O) ]2+(aq) + 2OH−(aq) → Fe(OH) (s)+ 6H O(l)

• state symbols 2 6 2 2

[Fe(H O) ]2+(aq) + 2OH−(aq) → Fe(OH) (H O) (s) +

26 2 2 4

2H2O(l)

Ignore missing square brackets

TE on cation that forms an insoluble hydroxide in

Test 1

State symbols conditional on correct species or ‘near

miss’ / non-ionic equation

Number

5(a)(iii) An answer that makes reference to the following point: (1)

Allow iron(III) hydroxide / iron(III) (ions) are

• Fe2+ is oxidised (to Fe3+ ) by oxygen / air formed by reaction with oxygen / air

TE on cation in Test 1

Allow just ‘the precipitate / it is oxidised by

oxygen / air’

Number

5(a)(iv) An answer that makes reference to the following point: (1)

Allow to prevent any other ions forming a

• to react with / remove any carbonate / sulfite / sulfate(IV) ions precipitate with barium ions / Ba2+

to eliminate the possibility of carbonate / sulfite / sulfate(IV)

ions

Number

5(b)(i) (2)

• (The cation in Y is) Cu2+ / copper(II) (1) Allow Cu+2

Ignore water ligands

Do not award just copper / Cu

• (The anion in Y is) Cl− / chloride (1) Do not award just ‘chlorine’ / Cl

Number

5(b)(ii) (1)

• [Cu(NH ) (H O) ]2+ Allow [Cu(NH ) ]2+

34 2 2 3 4

Allow [Co(NH ) ]2+ if Co2+ in (i)

Ignore missing square brackets

Number

5(b)(iii) A description that makes reference to the following (2)

points:

If Cl− / chloride / chlorine ion:

• add dilute (aqueous) ammonia (and stir the mixture) Allow add aqueous ammonia / NH3(aq)

(1) Do not award concentrated ammonia

• the precipitate dissolves (1) Conditional on use of ammonia

Ignore colourless solution

If Br− / bromide / bromine ion identified in Test 4:

precipitate is insoluble in dilute ammonia (1)

but soluble in concentrated ammonia (1)

If I− / iodide / iodine ion identified in test 4:

precipitate is insoluble (1)

in dilute and concentrated ammonia (1)

Do not award addition of concentrated sulfuric acid

(Total for Question 5 = 11 marks)

How to answer it

Analysis of Aqueous Solutions (Salts X and Y)

What this question tests

This question assesses qualitative analysis skills for inorganic ions (transition metal cations, sulfate ions, and halide ions), writing ionic equations with state symbols, understanding transition metal complex ion ligand exchange reactions, and explaining chemical observations such as redox changes and acidification reagents.

Part (a) — Analysis of Salt X

Identifying Ions, Ionic Equations, and Observations

✅ Correct Answers: (a)(i)

Cation in X: Fe²⁺ or iron(II) / Fe(II)
Anion in X: SO₄²⁻ or sulfate(VI)

Marks: 2

💡 Key Knowledge

Fe²⁺ aqueous ions react with NaOH to form a green gelatinous precipitate of Fe(OH)₂ .
Barium ions ( Ba²⁺ ) react with sulfate ions to produce a white precipitate of barium sulfate ( BaSO₄ ).

✅ Correct Answers: (a)(ii) Ionic Equation

Equation: Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s)

Alternative (complex ion form): [Fe(H₂O)₆]²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(H₂O)₄(s) + 2H₂O(l)

Marks: 2 (1 for species & balancing, 1 for correct state symbols)

❌ Common Errors

Forgetting state symbols or writing incorrect ones (e.g., writing (aq) for the solid precipitate). State symbols are conditional on correct or near-correct species.
Writing sulfite ( SO₃²⁻ ) instead of sulfate for the anion.

✅ Correct Answers: (a)(iii) & (a)(iv)

(a)(iii) Reason for colour change: Fe²⁺ is oxidised (to Fe³⁺ ) by oxygen / air in the atmosphere.
Marks: 1
(a)(iv) Why dilute HCl is needed: To react with and remove interfering ions (such as carbonate, sulfite, or sulfate(IV) ions) that would also form a white precipitate with barium ions.
Marks: 1

🧠 Exam Technique

When asked why an acid is added before a silver nitrate or barium chloride test, always state that it removes interfering ions which would otherwise give a false positive result (precipitation).

Part (b) — Analysis of Salt Y

Transition Metal Complexes and Halide Confirmation

✅ Correct Answers: (b)(i) & (b)(ii)

Cation in Y: Cu²⁺ or copper(II) (Marks: 2)
Anion in Y: Cl⁻ or chloride (Marks: 2 total with ii)
(b)(ii) Complex ion formula: [Cu(NH₃)₄(H₂O)₂]²⁺
(Note: [Cu(NH₃)₄]²⁺ is also accepted)
Marks: 1

💡 Key Knowledge

Aqueous Cu²⁺ gives a pale blue precipitate with dilute NH₃ , which redissolves in excess NH₃ via ligand exchange to form a deep blue solution.
Silver nitrate acidified with nitric acid tests for halides. Chloride gives a white precipitate ( AgCl ).

✅ Correct Answers: (b)(iii) Confirming Chloride

Reagent: Add dilute (aqueous) ammonia ( NH₃(aq) ).
Observation: The white precipitate dissolves (forming a colourless solution).

Marks: 2 (1 for reagent, 1 for observation)

❌ Common Errors

Using concentrated ammonia instead of dilute ammonia when distinguishing halide precipitates. Concentrated ammonia is used to confirm bromide ( Br⁻ ), whereas chloride dissolves in dilute ammonia.
Writing just "chlorine" or "Cl" instead of the chloride ion ( Cl⁻ ).

Topics

Inorganic Chemistry · Core Practicals · Core Practical 7: Identify unknown organic liquids and inorganic solids · Topic 4: Inorganic Chemistry and the Periodic Table · Topic 15: Transition Metals

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.