Edexcel A-Level Chemistry AS Paper 1, June 2019: Question 5

4 marks · Medium difficulty · Short Open Response

Calculate the percentage by mass of iron in iron(II) sulfate, describe the chemical test for sulfate ions, and calculate the molar mass of Mohr's salt.

Practise this question

Question

Question 5 about iron(II) salts. Part (a) is a multiple-choice question asking for the percentage by mass of iron in anhydrous iron(II) sulfate, FeSO4, with options A (21.3%), B (35.1%), C (36.7%), and D (53.8%). Part (b) asks to describe a chemical test and expected result to show sulfate ions are present in a solution of iron(II) sulfate. Part (c) is a multiple-choice question asking for the molar mass in g mol-1 of Mohr's salt, (NH4)2Fe(SO4)2·6H2O, with options A (392.0), B (312.0), C (302.0), and D (284.0).
Question text

5 This question is about iron(II) salts.

(a) What is the percentage by mass of iron in anhydrous iron(II) sulfate, FeSO4, to

3 significant figures?

(1)

A 21.3%

B 35.1%

C 36.7%

D 53.8%

(b) Describe a chemical test, and the expected result, to show that sulfate ions are

present in a solution of iron(II) sulfate in water.

(2)

(c) Mohr’s salt is another compound containing iron(II) ions.

It has the formula (NH4)2Fe(SO4)2. 6H2O.

What is the molar mass, in g mol−1, of Mohr’s salt?

(1)

A 392.0

B 312.0

C 302.0

D 284.0

(Total for Question 5 = 4 marks)

Mark scheme

Show the mark scheme Mark scheme for Question 5. Part (a) indicates the correct answer is C (36.7%) with 1 mark. Part (b) awards 2 marks: one for adding hydrochloric or nitric acid followed by barium chloride or barium nitrate solution, and one for observing a white precipitate or white solid. Part (c) indicates the correct answer is A (392.0) with 1 mark.

How to answer it

Iron(II) Salts Study Guide

What this question tests

This question assesses core foundational AS Chemistry quantitative and analytical skills: calculating percentage composition by mass, recalling and applying standard inorganic qualitative chemical tests (specifically for sulfate ions), and determining molar masses for complex hydrated formula units containing water of crystallisation and ammonium groups.

Question Part (a)

Percentage by Mass Calculation

✅ Correct Answer

C (36.7%)

Awarded 1 mark for selecting C.

📐 Step-by-Step Calculation

  1. Find the molar mass of FeSO₄:
    Ar(Fe) = 55.8, Ar(S) = 32.1, Ar(O) = 16.0 × 4 = 64.0
  2. Total Molar Mass = 55.8 + 32.1 + 64.0 = 151.9 g mol⁻¹
  3. Calculate percentage of iron:
    (55.8 / 151.9) × 100 = 36.735...%
  4. Round to 3 significant figures: 36.7%

❌ Common Errors & Distractors

  • A (21.3%): Calculated mistakenly using the atomic number of iron instead of its relative atomic mass.
  • B (35.1%): Calculated by mistakenly using atomic numbers for all elements.
  • D (53.8%): Calculated using only the mass contribution of sulfur and oxygen, omitting iron.
Question Part (b)

Qualitative Test for Sulfate Ions

✅ Correct Answer

Reagent: Dilute hydrochloric acid (or nitric acid) followed by barium chloride (or barium nitrate) solution.
Result: White precipitate (or white solid).

Awarded 2 marks (1 for correct reagents with acid, 1 for the white precipitate observation).

💡 Key Knowledge

  • Adding an acid first (like HCl or HNO₃) is crucial to react with and remove interfering impurity ions (such as carbonates or sulfites) that would also form a white precipitate with barium ions. Do NOT use sulfuric acid, as it introduces sulfate ions!
  • Barium ions (Ba²⁺) react with sulfate ions (SO₄²⁻) to form an insoluble white precipitate of barium sulfate: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

🧠 Exam Technique & Guidance

  • Dependency: Mark 2 (the observation) is dependent on mark 1, except if just "barium ions" or "Ba²⁺" was given as the reagent.
  • Never state just "add barium". You must specify a soluble salt solution like barium chloride or barium nitrate.
Question Part (c)

Molar Mass of Complex Hydrated Salts (Mohr's Salt)

✅ Correct Answer

A (392.0 g mol⁻¹)

Awarded 1 mark for selecting A.

📐 Step-by-Step Calculation

  1. Break down the formula: (NH₄)₂Fe(SO₄)₂ · 6H₂O
  2. Ammonium groups: 2 × [14.0 + (4 × 1.0)] = 2 × 18.0 = 36.0
  3. Iron: 1 × 55.8 = 55.8
  4. Sulfate groups: 2 × [32.1 + (4 × 16.0)] = 2 × 96.1 = 192.2
  5. Water of crystallisation: 6 × [(2 × 1.0) + 16.0] = 6 × 18.0 = 108.0
  6. Total Molar Mass = 36.0 + 55.8 + 192.2 + 108.0 = 392.0 g mol⁻¹

❌ Common Errors & Traps

  • B (312.0): Mistakenly multiplied only the hydrogens in water by 6, rather than the entire H₂O molecule unit.
  • C (302.0): Forgot to multiply the water of crystallisation mass component by 6 entirely.
  • D (284.0): Completely ignored the · 6H₂O hydration part of the formula.

Topics

Physical Chemistry · Inorganic Chemistry · Topic 5: Formulae, Equations and Amounts of Substance · Topic 4: Inorganic Chemistry and the Periodic Table

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.