OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2021: Question 15
1 mark · Medium difficulty · Multiple Choice
Identify the correct test results for the hydrated double salt ammonium iron(II) sulfate from a given table of three test reactions.
Practise this questionQuestion
Question text
15 NH4Fe(SO4)2•12H2O is a hydrated ‘double salt’.
A student analyses this double salt using test tube tests.
Which row(s) gives/give correct result(s) for the stated test?
Test Results
1 Reaction with cold NaOH(aq) Green precipitate
2 Reaction with Ba(NO3)2(aq) White precipitate
3 Reaction with warm NaOH(aq) Red-brown precipitate and an alkaline gas
A 1, 2 and 3
B Only 1 and 2
C Only 2 and 3
D Only 1
Your answer [1]
Mark scheme
Show the mark scheme
15 C 1 AO2.3
Total 15
How to answer it
Analysis of a Hydrated Double Salt
What this question tests
This question assesses your practical knowledge of qualitative inorganic analysis (test tube tests). You need to identify specific ions present in a complex formula ( NH₄Fe(SO₄)₂•12H₂O ) and recall the characteristic observations for transition metal precipitation, sulfate confirmation, and ammonium gas evolution.
Question Analysis & Breakdown
Evaluating the Double Salt Formula
The compound given is NH₄Fe(SO₄)₂•12H₂O . Breaking down its constituent ions:
- Cations: Ammonium ions ( NH₄⁺ ) and Iron(II) / Iron(III) depending on oxidation state. Here, iron is present as Fe³⁺ (as deduced from the stoichiometry and standard double salt compositions like Mohr's salt or iron alum).
- Anions: Sulfate ions ( SO₄²⁻ )
- Water of crystallisation: 12H₂O
✅ Correct Answer: C (Only 2 and 3)
Let's evaluate each row systematically to see why Row 2 and Row 3 are correct, while Row 1 is incorrect.
💡 Key Knowledge: Ion Tests
- Sulfate test: Add acidified Ba²⁺(aq) to form a white precipitate of BaSO₄ .
- Iron(III) + cold NaOH: Forms a rust brown (or red-brown) precipitate, not green. Green precipitates are characteristic of Fe²⁺ .
- Ammonium test: Warm with NaOH(aq) to release ammonia gas ( NH₃ ), which turns damp red litmus paper blue (alkaline gas).
🧠 Exam Technique
Tackle multiple-choice questions on qualitative analysis by evaluating rows independently or eliminating options. Spotting that Row 1 describes an Fe²⁺ precipitate instead of an Fe³⁺ precipitate immediately rules out options A, B, and D!
❌ Common Errors
- Confusing the colours of iron precipitates: Fe²⁺(aq) gives a green precipitate (often turning brown on the surface due to oxidation), whereas Fe³⁺(aq) gives a distinct red-brown precipitate instantly.
- Forgetting that testing for NH₄⁺ requires gentle warming with sodium hydroxide, as ammonia gas is soluble in cold water.
Topics
Module 5: Physical chemistry and transition elements · Practical Activity Groups · 5.3 Transition elements · PAG 4: Qualitative analysis of ions
Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.