AQA A-Level Chemistry Paper 3, 2019: Question 16
1 mark · Easy difficulty · Multiple Choice
Identify which salt forms a white precipitate with both aqueous silver nitrate and dilute sulfuric acid.
Practise this questionQuestion
Question text
16 An aqueous solution of a salt gives a white precipitate when mixed with
aqueous silver nitrate and when mixed with dilute sulfuric acid.
Which could be the formula of the salt?
[1 mark]
A BaCl2
B (NH4)2SO4
C KCl
D Sr(NO3)2
Mark scheme
Show the mark scheme
16 A 1
How to answer it
Precipitation Tests: Identifying Halides & Alkaline Earth Metals
This question assesses knowledge of qualitative inorganic analysis from Group 7 (Halide ions) and Group 2 (Alkaline earth metals):
- Using acidified silver nitrate ( AgNO₃ ) to identify halide ions via formation of insoluble silver halides.
- Using dilute sulfuric acid ( H₂SO₄ ) / sulfate ions ( SO₄²⁻ ) to identify Group 2 cations via insoluble sulfates.
- Applying solubility rules to deduce the identity of an ionic compound meeting multiple criteria.
Deducing the Salt from Dual Precipitation Reactions
1 Mark
✅ Correct Answer
A: BaCl₂
Reaction 1 (with AgNO₃):
Ag⁺(aq) + Cl⁻(aq) → AgCl(s) (white precipitate)
Reaction 2 (with H₂SO₄):
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) (white precipitate)
📐 Step-by-Step Option Elimination
| Option | + AgNO₃(aq) | + H₂SO₄(aq) |
|---|---|---|
| A: BaCl₂ | White ppt (AgCl) | White ppt (BaSO₄) |
| B: (NH₄)₂SO₄ | No ppt* | No ppt (no reaction) |
| C: KCl | White ppt (AgCl) | No ppt (K₂SO₄ soluble) |
| D: Sr(NO₃)₂ | No ppt (nitrates soluble) | White ppt (SrSO₄) |
*Ag₂SO₄ is sparingly soluble, but no precipitate forms in standard test concentrations.
💡 Key Knowledge
- Silver Halides:
- AgCl: White precipitate (soluble in dilute NH₃)
- AgBr: Cream precipitate (soluble in conc. NH₃)
- AgI: Yellow precipitate (insoluble in conc. NH₃)
- Group 2 Sulfate Solubility:
- Solubility decreases down Group 2: MgSO₄ (soluble) > CaSO₄ (sparingly) > SrSO₄ (insoluble) > BaSO₄ (insoluble).
- BaSO₄ is completely insoluble and appears as a dense white precipitate.
🧠 Exam Technique
- Deconstruct the clues independently:
- "White ppt with AgNO₃" immediately points to Cl⁻. This eliminates B and D in seconds.
- "White ppt with H₂SO₄" indicates an insoluble sulfate, typically Ba²⁺, Sr²⁺, or Pb²⁺. Between remaining options A (BaCl₂) and C (KCl), only Ba²⁺ forms an insoluble sulfate.
- Always test both cation and anion against every condition before finalizing your answer.
❌ Common Errors & Misconceptions
- Confusing Group 2 Trends: Students frequently mix up sulfate solubility with hydroxide solubility. Remember: Hydroxides become more soluble down Group 2, while sulfates become less soluble down Group 2.
- Selecting Option C (KCl): Students notice that chloride gives a white precipitate with AgNO₃, but forget that Group 1 sulfates (like K₂SO₄) are completely soluble in water, so no precipitate forms with H₂SO₄.
- Selecting Option D (Sr(NO₃)₂): Students recognize that Sr²⁺ forms a white precipitate with sulfuric acid (SrSO₄), but overlook that all nitrates (including AgNO₃ and Sr(NO₃)₂) are soluble, so no precipitate forms with AgNO₃.
Topics
Inorganic Chemistry · Required Practicals · 3.2.2 Group 2, The Alkaline Earth Metals · 3.2.3 Group 7(17), The Halogens · Required Practical 4: Carry out simple test-tube reactions to identify Cations and Anions
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.