Example Questions Cation / Anion (Copy)

Example 1: Cation and Anion Identification in a Mixed Solution

Question:
You are given an unknown solution that contains a mixture of two cations and one anion. You are also given the following reagents:

• Sodium hydroxide (NaOH)
• Barium chloride (BaCl₂)
• Silver nitrate (AgNO₃)
• Dilute hydrochloric acid (HCl)

Describe the step-by-step process you would use to identify the cations and anion in the solution. After performing the tests, you find the following observations:

1. No precipitate forms when NaOH is added.
2. A white precipitate forms when BaCl₂ is added.
3. A yellow precipitate forms when AgNO₃ is added.

Identify the cations and anion in the solution and explain your reasoning.

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Solution:

1. Test for Cations:
   • Observation: No precipitate forms when NaOH is added.
     • This indicates that the cation is not Copper(II) (Cu²⁺), Iron(II) (Fe²⁺), Iron(III) (Fe³⁺), Aluminum (Al³⁺), or Zinc (Zn²⁺), all of which form precipitates with NaOH. Therefore, the cation is likely Ammonium (NH₄⁺), as ammonium does not form a precipitate with NaOH but releases ammonia gas on heating.
   • Additional Check: To confirm Ammonium, you can heat the solution with NaOH. If ammonia gas is produced and turns red litmus paper blue, you have confirmed Ammonium ions (NH₄⁺).
2. Test for Anion (Sulfate):
   • Observation: A white precipitate forms when Barium chloride (BaCl₂) is added.
     • This indicates the presence of Sulfate ions (SO₄²⁻). The white precipitate is Barium sulfate (BaSO₄), which forms only in the presence of sulfate ions.
3. Test for the Second Cation (Halide):
   • Observation: A yellow precipitate forms when Silver nitrate (AgNO₃) is added.
     • A yellow precipitate indicates the presence of Iodide ions (I⁻). The yellow precipitate is Silver iodide (AgI), which is insoluble in ammonia.

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Conclusion:

• The cations in the unknown solution are Ammonium (NH₄⁺) and Silver (Ag⁺).
• The anion is Sulfate (SO₄²⁻), and the Iodide (I⁻) is present as the second anion.

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Example 2: Ambiguous Precipitation Reactions

Question:
An unknown solution is tested with the following reagents:

• Sodium hydroxide (NaOH)
• Ammonium chloride (NH₄Cl)
• Potassium chromate (K₂CrO₄)

The following observations were made:

1. A pale yellow precipitate forms when NaOH is added.
2. The precipitate dissolves when excess NaOH is added.
3. The solution turns red when ammonium chloride is added.

What is the cation in the unknown solution? Explain your reasoning.

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Solution:

1. Test with NaOH:
   • Observation: A pale yellow precipitate forms when NaOH is added.
     • The pale yellow precipitate suggests the presence of Chromium(III) ions (Cr³⁺). Chromium(III) forms a yellow precipitate with NaOH, but it is amphoteric and can dissolve in excess NaOH to form a soluble complex.
   • Observation: The precipitate dissolves in excess NaOH.
     • This confirms that the cation is Chromium(III) (Cr³⁺) because Cr(OH)₃ dissolves in excess NaOH to form the complex ion [Cr(OH)₄]⁻.
2. Test with Ammonium chloride:
   • Observation: The solution turns red when ammonium chloride (NH₄Cl) is added.
     • The red color indicates the presence of Chromium(III) ions (Cr³⁺). The addition of NH₄Cl to chromium solutions creates an acidic environment, leading to the formation of [CrCl₄]²⁻, which is red.

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Conclusion:

• The cation in the unknown solution is Chromium(III) (Cr³⁺).

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Example 3: Complicated Halide Test

Question:
You are given an unknown solution that may contain Chlorides, Bromides, or Iodides. You perform the following steps:

1. Add silver nitrate (AgNO₃) to the solution, and a white precipitate forms.
2. You then add ammonia (NH₃) to the precipitate, and it dissolves completely.
3. You add concentrated ammonia (NH₃), and the precipitate reappears.

What is the halide present in the solution?

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Solution:

1. Silver Nitrate Test:
   • Observation: A white precipitate forms when AgNO₃ is added. This suggests the presence of Chloride ions (Cl⁻), Bromide ions (Br⁻), or Iodide ions (I⁻), as all of these halides form precipitates with silver nitrate.
     • Chloride (Cl⁻): Forms white precipitate of AgCl.
     • Bromide (Br⁻): Forms cream precipitate of AgBr.
     • Iodide (I⁻): Forms yellow precipitate of AgI.
2. Ammonia (NH₃) Addition:
   • Observation: The white precipitate dissolves completely in ammonia.
     • Silver chloride (AgCl) is soluble in dilute ammonia, whereas AgBr and AgI are not soluble in dilute ammonia.
     • This suggests that the precipitate is AgCl (Silver chloride), meaning Chloride ions (Cl⁻) are present.
3. Concentrated Ammonia (NH₃) Addition:
   • Observation: The precipitate reappears after adding concentrated ammonia.
     • This could indicate that the original solution contains Silver chloride (AgCl), which re-forms after excess ammonia is added, as the initial solution was not fully saturated with ammonia.

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Conclusion:

• The halide in the solution is Chloride (Cl⁻).

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Example 4: Mixed Solutions with Interfering Elements

Question:
You are given an unknown solution that may contain Aluminum (Al³⁺), Calcium (Ca²⁺), and Sulfate (SO₄²⁻). You perform the following tests:

1. A white precipitate forms when sodium hydroxide (NaOH) is added.
2. The precipitate dissolves in excess NaOH to form a colorless solution.
3. A white precipitate forms when barium chloride (BaCl₂) is added.
4. The precipitate does not dissolve in hydrochloric acid (HCl).

What are the cations and anions present in the solution?

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Solution:

1. Sodium Hydroxide Test:
   • Observation: A white precipitate forms when NaOH is added.
     • This suggests the presence of Calcium (Ca²⁺) or Aluminum (Al³⁺) because both can form a white precipitate with NaOH.
     • The white precipitate is likely Al(OH)₃ for Aluminum, as Calcium hydroxide (Ca(OH)₂) is slightly soluble in water and will not give a precipitate under normal conditions.
2. Excess Sodium Hydroxide:
   • Observation: The precipitate dissolves in excess NaOH to form a colorless solution.
     • This confirms the presence of Aluminum ions (Al³⁺) because Al(OH)₃ is amphoteric and dissolves in excess NaOH to form [Al(OH)₄]⁻.
3. Barium Chloride Test:
   • Observation: A white precipitate forms when BaCl₂ is added.
     • This indicates the presence of Sulfate ions (SO₄²⁻), as BaSO₄ forms a white precipitate when Barium chloride is added to solutions containing sulfate ions.
4. HCl Test:
   • Observation: The precipitate does not dissolve in HCl.
     • BaSO₄ (Barium sulfate) is insoluble in HCl, confirming that Sulfate ions (SO₄²⁻) are present.

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Conclusion:

• The cation in the solution is Aluminum (Al³⁺).
• The anion in the solution is Sulfate (SO₄²⁻).

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Final Tips:

• Always write down observations carefully and sequentially.
• Do not rush the flame test: Make sure to clean the wire properly before each test.
• Ensure that you add excess reagents when necessary to identify dissolving precipitates or confirming the presence of specific ions.
• Avoid assumptions: Always base your answer on concrete test results, not on theoretical predictions.

Example 1:

You are given an unknown solution that contains a mixture of cations and anions. You perform the following tests:

1. Add sodium hydroxide (NaOH) to the solution and observe a green precipitate.
2. The precipitate dissolves when excess NaOH is added.
3. When barium chloride (BaCl₂) is added to the solution, a white precipitate forms.
4. When silver nitrate (AgNO₃) is added, a cream precipitate forms.
5. The cream precipitate does not dissolve in ammonia.

What are the cations and anions in the solution?

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Solution:

1. Green precipitate with NaOH indicates the presence of Iron(II) ions (Fe²⁺), which form a green precipitate of Fe(OH)₂.
2. The precipitate dissolves in excess NaOH, confirming that Fe²⁺ is present since Fe(OH)₂ is amphoteric.
3. The white precipitate with BaCl₂ suggests the presence of Sulfate ions (SO₄²⁻), as BaSO₄ forms a white precipitate.
4. The cream precipitate with AgNO₃ indicates the presence of Bromide ions (Br⁻), as AgBr forms a cream precipitate.
5. The cream precipitate does not dissolve in ammonia, confirming Bromide ions (Br⁻) because AgBr is insoluble in ammonia.

Cations: Iron(II) (Fe²⁺)
Anions: Sulfate (SO₄²⁻) and Bromide (Br⁻)

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Example 2:

You are given an unknown solution that may contain Copper(II) ions (Cu²⁺), Calcium ions (Ca²⁺), and Sulfate ions (SO₄²⁻). You perform the following tests:

1. When sodium hydroxide (NaOH) is added to the solution, a blue precipitate forms.
2. The blue precipitate dissolves in excess NaOH to form a colorless solution.
3. When barium chloride (BaCl₂) is added, a white precipitate forms.
4. The precipitate does not dissolve in hydrochloric acid (HCl).
5. The solution turns milky when dilute hydrochloric acid (HCl) is added.

What are the cations and anions in the solution?

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Solution:

1. The blue precipitate that dissolves in excess NaOH suggests the presence of Copper(II) ions (Cu²⁺), as Cu(OH)₂ forms a blue precipitate and dissolves in excess NaOH to form a colorless [Cu(OH)₄]²⁻ complex.
2. The white precipitate with BaCl₂ confirms the presence of Sulfate ions (SO₄²⁻), as BaSO₄ forms a white precipitate.
3. The precipitate does not dissolve in HCl, confirming BaSO₄ is present, as BaSO₄ is insoluble in hydrochloric acid.
4. The solution turns milky when dilute hydrochloric acid is added, confirming the presence of Calcium ions (Ca²⁺), as calcium reacts with CO₃²⁻ (likely present as part of the Calcium carbonate impurity).

Cations: Copper(II) (Cu²⁺), Calcium (Ca²⁺)
Anions: Sulfate (SO₄²⁻)

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Example 3:

An unknown solution is tested with the following reagents:

1. Add sodium hydroxide (NaOH), and a white precipitate forms that is insoluble in excess NaOH.
2. Add ammonium chloride (NH₄Cl), and a yellow precipitate forms.
3. Add barium chloride (BaCl₂), and no precipitate forms.
4. Add silver nitrate (AgNO₃), and a white precipitate forms.

What are the cations and anions in the solution?

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Solution:

1. The white precipitate that is insoluble in excess NaOH indicates the presence of Calcium ions (Ca²⁺), as Ca(OH)₂ forms a white precipitate that is insoluble in excess NaOH.
2. The yellow precipitate formed with ammonium chloride (NH₄Cl) suggests the presence of Chromium(III) ions (Cr³⁺), as Cr(OH)₃ forms a yellow precipitate and reacts with ammonium chloride to form a yellow solution.
3. The lack of a precipitate when BaCl₂ is added suggests that Sulfate ions (SO₄²⁻) are not present, as no precipitate of BaSO₄ is observed.
4. The white precipitate with AgNO₃ indicates the presence of Chloride ions (Cl⁻), as AgCl forms a white precipitate.

Cations: Calcium (Ca²⁺), Chromium(III) (Cr³⁺)
Anion: Chloride (Cl⁻)

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Example 4:

You perform the following tests on an unknown solution:

1. Add sodium hydroxide (NaOH), and a white precipitate forms.
2. Add excess sodium hydroxide, and the precipitate dissolves to give a colorless solution.
3. Add potassium chromate (K₂CrO₄), and a yellow precipitate forms.
4. Add dilute hydrochloric acid (HCl), and no observable change occurs.

What are the cations and anions in the solution?

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Solution:

1. The white precipitate that dissolves in excess NaOH suggests the presence of Zinc ions (Zn²⁺), as Zn(OH)₂ forms a white precipitate that dissolves in excess NaOH to form a colorless [Zn(OH)₄]²⁻ complex.
2. The yellow precipitate that forms with potassium chromate (K₂CrO₄) suggests the presence of Lead(II) ions (Pb²⁺), as PbCrO₄ forms a yellow precipitate.
3. The lack of any observable change with dilute HCl indicates that PbCrO₄ does not react with dilute acid.

Cations: Zinc (Zn²⁺), Lead(II) (Pb²⁺)
Anion: Chromate (CrO₄²⁻)

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Example 5:

An unknown solution is tested with the following reagents:

1. Add sodium hydroxide (NaOH), and a brown precipitate forms.
2. Add excess NaOH, and the precipitate does not dissolve.
3. Add barium chloride (BaCl₂), and no precipitate forms.
4. Add silver nitrate (AgNO₃), and a cream precipitate forms.

What are the cations and anions in the solution?

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Solution:

1. The brown precipitate that does not dissolve in excess NaOH suggests the presence of Iron(III) ions (Fe³⁺), as Fe(OH)₃ forms a brown precipitate that is insoluble in excess NaOH.
2. The lack of a precipitate when BaCl₂ is added suggests that Sulfate ions (SO₄²⁻) are not present.
3. The cream precipitate formed with AgNO₃ indicates the presence of Bromide ions (Br⁻), as AgBr forms a cream precipitate.

Cation: Iron(III) (Fe³⁺)
Anion: Bromide (Br⁻)

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Example 6:

An unknown solution is tested with the following reagents:

1. A white precipitate forms when sodium hydroxide (NaOH) is added.
2. The precipitate dissolves in excess NaOH, forming a colorless solution.
3. Add barium chloride (BaCl₂), and a white precipitate forms.
4. Add ammonia (NH₃), and a green solution forms.

What are the cations and anions in the solution?

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Solution:

1. The white precipitate that dissolves in excess NaOH and forms a colorless solution suggests the presence of Aluminum ions (Al³⁺), as Al(OH)₃ forms a white precipitate and dissolves in excess NaOH to form a colorless solution of [Al(OH)₄]⁻.
2. The white precipitate that forms with BaCl₂ indicates the presence of Sulfate ions (SO₄²⁻), as BaSO₄ forms a white precipitate.
3. The green solution that forms when ammonia (NH₃) is added suggests the presence of Chromium(III) ions (Cr³⁺), as Cr(OH)₃ forms a green precipitate and dissolves in ammonia to form a green solution.

Cations: Aluminum (Al³⁺), Chromium(III) (Cr³⁺)
Anion: Sulfate (SO₄²⁻)

Example 7:

You are given an unknown solution and perform the following tests:

1. Add sodium hydroxide (NaOH), and a blue precipitate forms.
2. The precipitate dissolves when excess NaOH is added to give a colorless solution.
3. Add barium chloride (BaCl₂), and a white precipitate forms.
4. When silver nitrate (AgNO₃) is added, a white precipitate forms.
5. The precipitate formed by AgNO₃ dissolves in ammonia.

What are the cations and anions in the solution?

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Solution:

1. The blue precipitate that dissolves in excess NaOH suggests the presence of Copper(II) ions (Cu²⁺), as Cu(OH)₂ forms a blue precipitate and dissolves in excess NaOH to form a colorless [Cu(OH)₄]²⁻ complex.
2. The white precipitate that forms with BaCl₂ suggests the presence of Sulfate ions (SO₄²⁻), as BaSO₄ forms a white precipitate.
3. The white precipitate formed with AgNO₃ suggests the presence of Chloride ions (Cl⁻), as AgCl forms a white precipitate.
4. The fact that the precipitate dissolves in ammonia confirms that the anion is Chloride (Cl⁻), as AgCl is soluble in ammonia.

Cations: Copper(II) (Cu²⁺)
Anion: Chloride (Cl⁻)

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Example 8:

An unknown solution is tested with the following reagents:

1. Add sodium hydroxide (NaOH), and a green precipitate forms.
2. The precipitate turns brown upon standing.
3. Add barium chloride (BaCl₂), and a white precipitate forms.
4. Add silver nitrate (AgNO₃), and a yellow precipitate forms.

What are the cations and anions in the solution?

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Solution:

1. The green precipitate that turns brown upon standing suggests the presence of Iron(II) ions (Fe²⁺). Fe(OH)₂ forms a green precipitate that oxidizes to Fe(OH)₃, turning it brown.
2. The white precipitate that forms with BaCl₂ suggests the presence of Sulfate ions (SO₄²⁻), as BaSO₄ forms a white precipitate.
3. The yellow precipitate formed with AgNO₃ indicates the presence of Iodide ions (I⁻), as AgI forms a yellow precipitate.

Cations: Iron(II) (Fe²⁺)
Anion: Sulfate (SO₄²⁻) and Iodide (I⁻)

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Example 9:

You perform the following tests on an unknown solution:

1. Add sodium hydroxide (NaOH), and a white precipitate forms.
2. The precipitate dissolves in excess NaOH to form a colorless solution.
3. Add potassium chromate (K₂CrO₄), and a yellow precipitate forms.
4. Add barium chloride (BaCl₂), and no precipitate forms.

What are the cations and anions in the solution?

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Solution:

1. The white precipitate that dissolves in excess NaOH suggests the presence of Zinc ions (Zn²⁺), as Zn(OH)₂ forms a white precipitate and dissolves in excess NaOH to form a colorless [Zn(OH)₄]²⁻ complex.
2. The yellow precipitate formed with potassium chromate (K₂CrO₄) suggests the presence of Lead(II) ions (Pb²⁺), as PbCrO₄ forms a yellow precipitate.
3. The lack of a precipitate when BaCl₂ is added suggests that Sulfate ions (SO₄²⁻) are not present, as BaSO₄ would have formed a white precipitate.

Cation: Zinc (Zn²⁺)
Anion: Chromate (CrO₄²⁻)

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Example 10:

You are given an unknown solution and perform the following tests:

1. Add sodium hydroxide (NaOH), and a white precipitate forms.
2. The precipitate does not dissolve in excess NaOH.
3. Add potassium chromate (K₂CrO₄), and no precipitate forms.
4. Add silver nitrate (AgNO₃), and a white precipitate forms.
5. The white precipitate does not dissolve in ammonia.

What are the cations and anions in the solution?

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Solution:

1. The white precipitate that does not dissolve in excess NaOH suggests the presence of Calcium ions (Ca²⁺), as Ca(OH)₂ forms a white precipitate and is only slightly soluble in water.
2. The lack of a precipitate when K₂CrO₄ is added suggests that Sulfate ions (SO₄²⁻) are not present, as BaSO₄ would form a white precipitate.
3. The white precipitate formed with AgNO₃ indicates the presence of Chloride ions (Cl⁻), as AgCl forms a white precipitate.
4. The fact that the precipitate does not dissolve in ammonia confirms that the anion is Chloride (Cl⁻), as AgCl is insoluble in ammonia.

Cation: Calcium (Ca²⁺)
Anion: Chloride (Cl⁻)

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Example 11:

You are given an unknown solution and perform the following tests:

1. Add sodium hydroxide (NaOH), and a brown precipitate forms.
2. The precipitate does not dissolve in excess NaOH.
3. Add silver nitrate (AgNO₃), and a yellow precipitate forms.
4. Add barium chloride (BaCl₂), and no precipitate forms.

What are the cations and anions in the solution?

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Solution:

1. The brown precipitate that does not dissolve in excess NaOH suggests the presence of Iron(III) ions (Fe³⁺), as Fe(OH)₃ forms a brown precipitate that is insoluble in excess NaOH.
2. The yellow precipitate formed with AgNO₃ indicates the presence of Iodide ions (I⁻), as AgI forms a yellow precipitate.
3. The lack of a precipitate with BaCl₂ indicates that Sulfate ions (SO₄²⁻) are not present, as BaSO₄ would have formed a white precipitate.

Cation: Iron(III) (Fe³⁺)
Anion: Iodide (I⁻)

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Example 12:

An unknown solution is tested with the following reagents:

1. Add sodium hydroxide (NaOH), and a white precipitate forms.
2. The precipitate dissolves in excess NaOH to form a colorless solution.
3. Add ammonium chloride (NH₄Cl), and a yellow precipitate forms.
4. Add silver nitrate (AgNO₃), and a white precipitate forms.

What are the cations and anions in the solution?

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Solution:

1. The white precipitate that dissolves in excess NaOH suggests the presence of Zinc ions (Zn²⁺), as Zn(OH)₂ forms a white precipitate and dissolves in excess NaOH to form a colorless [Zn(OH)₄]²⁻ complex.
2. The yellow precipitate formed with ammonium chloride (NH₄Cl) suggests the presence of Chromium(III) ions (Cr³⁺), as Cr(OH)₃ forms a yellow precipitate and reacts with ammonium chloride to form a yellow solution.
3. The white precipitate formed with AgNO₃ suggests the presence of Chloride ions (Cl⁻), as AgCl forms a white precipitate.

Cations: Zinc (Zn²⁺), Chromium(III) (Cr³⁺)
Anion: Chloride (Cl⁻)

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