OCR A-Level Chemistry Unified chemistry (03), June 2023: Question 4

10 marks · Medium difficulty · Structured Questions

Calculate the masses of reactants needed to prepare a Tutton's salt, explain crystal growth conditions, and deduce the formulae of transition metal complex ions, precipitates, and test for sulfate ions.

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Question text

4 Tutton’s salts are ‘double salts’ with the formula X2Y(ZO4)2• 6H2O.

A Tutton’s salt contains two cations: X+ and Y2+.

• X+ can be an ion of the Group 1 elements K, Rb, Cs or Fr, or an ammonium ion.

• Y2+ can be a 2+ ion of magnesium or an ion of most of the transition elements in Period 4.

• Z can be S or Cr.

(NH4)2Cu(SO4)2• 6H2O is an example of a Tutton’s salt.

(a) Predict the formula of a Tutton’s salt containing different ions from (NH4)2Cu(SO4)2• 6H2O.

… [1]

(b) A student prepares a sample of the Tutton’s salt, (NH4)2Cu(SO4)2• 6H2O using the method

shown below.

Step 1 Dissolve 0.025 mol of ammonium sulfate and 0.025 mol of hydrated copper(II)

sulfate, CuSO • 5H O, in water and make up to 50 cm3.

Step 2 Boil the resulting mixture for 2 minutes and allow to cool.

Step 3 Allow the solvent to evaporate slowly. Pale blue crystals of the Tutton’s salt form.

(i) What masses are needed of ammonium sulfate and CuSO4• 5H2O?

mass of ammonium sulfate … g

mass of CuSO4• 5H2O … g

[2]

(ii) In Step 3, why does the student allow the solvent to evaporate and not boil off all the

solvent in Step 2?

… [1]

The student carries out two tests on this aqueous solution.

(i) The student adds an excess of aqueous ammonia to their aqueous solution of Tutton’s

salt. A deep blue solution forms.

The complex ion responsible for the deep blue solution has a molar mass of

167.5 g mol–1.

Suggest the formula of this complex ion.

… [1]

(ii) The student adds NaOH(aq) to the aqueous solution of Tutton’s salt and warms the

mixture.

A precipitate and a gas are formed.

Write the formulae of the precipitate and gas and suggest a test that could confirm the

identity of the gas.

Formula of precipitate …

Formula of gas …

Test to confirm the identity of the gas …

… [3]

(iii) How could the student carry out a test-tube test to confirm the anion in the Tutton’s salt?

… [2]

Mark scheme

Show the mark scheme

Question Answer Marks AO Guidance

element

4 (a) Any correct formula for X2Y(ZO4)2 • 6H2O 1 AO3.2

with suitable elements for X, Y and Z using information in stem: Suitable transition elements:

Ti, V, Cr, Mn, Fe, Co, Ni

• X can be K, Rb, Cs, Fr ONLY • Cu in in the Tutton’s salt in Q4

• Sc and Zn and not classified as

• Y can be Mg or a transition element in period 4: Ti → Ni transition elements

• Z must be Cr

Example: K2Mg(CrO4)2 • 6H2O

(b) (i) Mass (NH4)2SO4 = 3.3025 g 2 AO1.2 ALLOW 3.3, 3.30. 3.303

Mass CuSO4•5H2O = 6.24 g ALLOW 6.2

(b) (ii) • Prevents water of crystallisation from being removed 1 AO3.4 IGNORE all the water would be removed

• Anhydrous salt would form water is the solvent

• Prevents dehydration

IGNORE prevents decomposition

IGNORE increases the size of crystals

34 2 2

TAKE CARE with correct brackets, numbers and 2+ charge IGNORE [Cu(NH ) ]2+

H2O and NH3 can be in either order,

i.e. [ Cu(H O) (NH ) ]2+

22 3 4

Question Answer 18 Marks AO Guidance

element

IGNORE name: copper(II) hydroxide 3

--------------------------------------------- ALLOW charges on Cu AND OH

Formula of gas NH e.g. Cu2+(OH–)

IGNORE name: ammonia DO NOT ALLOW unbalanced charges.

--------------------------------------------- e.g. Cu(OH–)

Test for ammonia ---------------------------------------

Available only from a reasonable attempt for identifying the gas DO NOT ALLOW correct test for NH3

as NH , e.g. NH , NH +, NH , ammonia, ammonium based on incorrect ID of the gas

34 4 2

(Moist/damp) indicator/litmus (paper) turns blue NO ECF for a test on the wrong gas

(has to be test for NH3)

Moist/damp NOT required.

Initial colour of litmus NOT required but blue is CON DO NOT ALLOW bleaches indicator CON

BaCl2 / barium chloride (solution) 2 compounds

OR Ba(NO3)2 / barium nitrate (solution) ------------------------------------------

OR Ba2+ (solution/aq) / barium ions IGNORE test for other anions provided

they do NOT interfere with SO 2– test

e.g.

IGNORE addition of HCl/HNO /H+

Observation BUT DO NOT ALLOW H2SO4

white precipitate/ppt Interferes with SO 2– test

Only available from soluble Ba2+ reagent

IGNORE Ag+/AgNO after SO 2– test

ALLOW minor slips in formula of Ba2+ reagent, DO NOT ALLOW before SO 2– test

e.g. BaCl, BaNO3

IGNORE bubbling any gas through

limewater

IGNORE responses linked to CrO 2–

Not in Tutton’s salt that student prepares

How to answer it

Tutton's Salts & Transition Metal Chemistry

What this question tests

This question assesses your understanding of ionic formulas, stoichiometry calculations involving moles and molar mass, practical laboratory techniques for crystal growth, transition metal complex ions (ligand exchange and formulas), and qualitative chemical tests for cations (ammonium) and anions (sulfate).

Question Part (a)

Formulating a New Tutton's Salt

✅ Correct Answer

Any valid formula matching X₂Y(ZO₄)₂ • 6H₂O using allowed elements:

X: K, Rb, Cs, Fr or NH₄ (must be singly charged)
Y: Mg or Period 4 transition elements (Ti to Ni, excluding Sc and Zn)
Z: S or Cr

Example: K₂Mg(CrO₄)₂ • 6H₂O

💡 Key Knowledge

Scandium (Sc³⁺) and Zinc (Zn²⁺) do not form colored complex ions or variable oxidation states typical of transition metals because they lack partially filled d-subshells in their stable oxidation states.

Mark: 1 mark for any chemically sound formula following the structural pattern given in the stem.

Question Part (b)(i)

Calculation of Reagent Masses

📐 Step-by-Step Calculation

Find molar mass of (NH₄)₂SO₄:
(2 × 18.0) + 32.1 + (4 × 16.0) = 132.1 g mol⁻¹
Calculate mass of ammonium sulfate:
0.0250 mol × 132.1 g mol⁻¹ = 3.30 g (or 3.3025 g)
Find molar mass of CuSO₄ • 5H₂O:
63.5 + 32.1 + (4 × 16.0) + (5 × 18.0) = 249.6 g mol⁻¹
Calculate mass of copper(II) sulfate pentahydrate:
0.0250 mol × 249.6 g mol⁻¹ = 6.24 g

❌ Common Calculation Traps

Hydration oversight: Forgetting to include the mass of the 5 water molecules ( 5 × 18.0 ) when calculating the molar mass of hydrated copper(II) sulfate.
Significant figures: Rounding too aggressively too early in multi-step working. Stick to 3 significant figures to align with the 0.025 mol given in the prompt.

Marks: 2 marks total (1 mark for ammonium sulfate mass, 1 mark for hydrated copper sulfate mass).

Question Part (b)(ii)

Crystallisation Technique

🧠 Exam Technique & Correct Answer

Answer: Allows slow evaporation without removing the water of crystallisation / prevents the anhydrous salt from forming / prevents dehydration.

Boiling off all the solvent completely would decompose the hydrate structure, removing the essential 6H₂O water molecules required to form the Tutton's salt crystals.

❌ Common Errors

Avoid saying "it prevents water from being removed completely" vaguely, or stating "water is the solvent". Be precise: you must reference water of crystallisation or hydration.

Mark: 1 mark for linking evaporation control to retaining water of crystallisation.

Question Part (c)(i)

Ligand Substitution & Complex Ion Formula

✅ Correct Answer

[Cu(NH₃)₄(H₂O)₂]²⁺ (or [Cu(H₂O)₂_n(NH₃)₄]²⁺ variations)

Check molar mass: 63.5 + (4 × 17.0) + (2 × 18.0) = 63.5 + 68.0 + 36.0 = 167.5 g mol⁻¹, matching the prompt data perfectly.

💡 Key Knowledge

Excess aqueous ammonia undergoes a ligand substitution reaction with hexaaquacopper(II). Neutral water molecules are replaced by neutral ammonia molecules, keeping the overall 2+ charge of the complex identical.

Mark: 1 mark for correct formula with proper square brackets and 2+ charge.

Question Part (c)(ii)

Alkali Precipitation & Gas Identification

✅ Correct Answers

Formula of precipitate: Cu(OH)₂ (or Cu(OH)₂(H₂O)₄ )
Formula of gas: NH₃
Test to confirm gas: Hold moist/damp red litmus paper in the gas; it turns blue.

🧠 Exam Technique Insights

When asked for a test, examiners strictly award marks only if you state the initial state of the indicator (moist/damp) and the final observed colour change (turns blue). Never write "blue litmus stays blue" as it gains no credit.

Marks: 3 marks total (1 for precipitate formula, 1 for gas formula, 1 for gas test description).

Question Part (c)(iii)

Testing for Sulfate Anions

✅ Correct Answer

Reagent: Add aqueous barium chloride ( BaCl₂ ) or barium nitrate ( Ba(NO₃)₂ ).
Observation: A dense white precipitate forms ( BaSO₄ ).

❌ Common Errors to Avoid

Students frequently forget to add dilute acid ( HCl or HNO₃ ) beforehand in general protocols to eliminate false positives (like carbonates or sulfites). However, note that adding sulfuric acid ( H₂SO₄ ) is an automatic zero because it introduces sulfate ions directly!

Marks: 2 marks total (1 for correct soluble barium reagent, 1 for white precipitate observation).

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · Module 5: Physical chemistry and transition elements · Practical Activity Groups · 5.3 Transition elements · PAG 1: Moles determination · PAG 4: Qualitative analysis of ions · PAG 6: Synthesis of an organic solid · 3.1 The periodic table · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, Unified chemistry (03), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.