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FB 7, FB 8 and FB 9 are aqueous solutions, each containing cations and anions from those listed on pages 11 and 12 in the Qualitative Analysis Notes - CIE - A-Level Chemistry - Question 3 - 2010 - Paper 1
Question 3
FB 7, FB 8 and FB 9 are aqueous solutions, each containing cations and anions from those listed on pages 11 and 12 in the Qualitative Analysis Notes. At each stage ... show full transcript
Worked Solution & Example Answer:FB 7, FB 8 and FB 9 are aqueous solutions, each containing cations and anions from those listed on pages 11 and 12 in the Qualitative Analysis Notes - CIE - A-Level Chemistry - Question 3 - 2010 - Paper 1
Step 1
Suggest a reagent that would enable you to identify the presence of NH₄⁺ and describe how you would use the reagent in an appropriate test.
Answer
A suitable reagent to identify the presence of ammonium ion (NH₄⁺) is sodium hydroxide (NaOH). When NaOH is added to the solution containing NH₄⁺, it will react to produce ammonia gas (NH₃) upon heating. This gas can be recognized by its distinctive smell, and it will turn moist red litmus paper blue, indicating the presence of the ammonia gas.
To perform the test:
- Add a few drops of NaOH to the test solution (FB 7, FB 8, or FB 9).
- Heat the mixture gently.
- If ammonia is released, confirm its presence using red litmus paper.
Step 2
One or more of the solutions contains the sulfate ion, SO₄²⁻. Select reagents that would enable you to identify the presence of SO₄²⁻.
Answer
To identify the presence of sulfate ion (SO₄²⁻), barium chloride (BaCl₂) can be used. When BaCl₂ is added to a solution containing SO₄²⁻, a white precipitate of barium sulfate (BaSO₄) is formed.
For testing and distinguishing SO₄²⁻ from sulfite ion (SO₃²⁻):
- Add barium chloride to the solution being tested (FB 7, FB 8, or FB 9).
- If a white precipitate forms, add dilute hydrochloric acid (HCl).
- If the precipitate dissolves, it indicates that the ion present is a sulfate; if it remains, the ion present is sulfite.
Step 3
Step 4
Use aqueous sodium hydroxide and aqueous ammonia in separate tests to identify any cation (apart from NH₄⁺) present in FB 7, FB 8 and FB 9.
Answer
Results:
| Solution | Cation | Supporting Evidence |
|---|---|---|
| FB 7 | Fe²⁺ | Green precipitate formed with NaOH |
| FB 8 | Na⁺ | No reaction observed with NaOH |
| FB 9 | Ca²⁺ | White precipitate formed with NaOH |
Step 5
Step 6
Carry out the following tests on FB 10. Observe carefully at each stage and record all of your observations in the table.
Answer
Observations for Tests on FB 10:
- Heating FB 10 resulted in the solid turning black initially, indicating possible decomposition.
- Upon further heating, solid layers were observed, which might indicate different phases or compounds present.
Further tests for gases produced and any observations on solubility were noted.