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CHEMISTRY, M8 2016 HSC 13 MC

The flow chart shows the steps used to identify a sample of a substance.
 

If the substance is sodium sulfate, what should have been observed in Tests 1,2 and 3 ?
 

  Test 1 Test 2 Test 3
A.    Bright orange flame No bubbles White precipitate formed
B. Bright orange flame Bubbles No precipitate formed
C.  Blue-green flame No bubbles No precipitate formed
D. Blue-green flame Bubbles White precipitate formed
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`A`

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By Elimination:

→ Sodium produces a bright yellow-orange colour in a flame test (eliminate C and D)

→ No gas is produced when sulphate reacts with acid (eliminate B)

`=>A`

CHEMISTRY, M8 2017 HSC 2 MC

Which row of the table correctly matches an ion with its flame colour during a flame test?
 

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`D`

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Consider each option:

→ Barium ions produce blue-green (incorrect)

→ Calcium ions produce orange-red (incorrect)

→ Carbonate ions produce no colour (incorrect)

→ Copper ions produce blue-green (correct)

`=>D`

CHEMISTRY, M8 2020 HSC 22

A 0.1 mol L ¯1 solution of an unknown salt is to be analysed. The cation is one of magnesium, calcium or barium. The anion is one of chloride, acetate or hydroxide.

Outline a sequence of tests that could be performed in a school laboratory to confirm the identity of this salt solution. Include expected observations and a balanced chemical equation in your answer.   (5 marks)

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Cation:

→ The cation can be identified via a flame test. 

→ A pale green flame indicates barium, a brick red flame indicates calcium, and no flame colour observed indicates magnesium.
 

Anion:

→ To identify anion, add copper nitrate.

→ If a precipitate forms, it indicates the presence of hydroxide ions.

→ Next, add silver nitrate. If a white precipitate forms, this indicates chloride ions present. If no precipiate forms, it indicates acetate ions present.

\( \ce{Ag+(aq) + Cl-(aq) -> AgCl(s)} \)
 

Other answers

→ Identify anion by testing pH with universal indicator.

→ Interpret result: Neutral (`text{Cl}^-` present), slightly basic (`text{CH}_3text{COO}^-` present), very basic (`text{OH}^-` present).

Show Worked Solution

Cation:

→ The cation can be identified via a flame test. 

→ A pale green flame indicates barium, a brick red flame indicates calcium, and no flame colour observed indicates magnesium.
 

Anion:

→ To identify anion, add copper nitrate.

→ If a precipitate forms, it indicates the presence of hydroxide ions.

→ Next, add silver nitrate. If a white precipitate forms, this indicates chloride ions present. If no precipiate forms, it indicates acetate ions present.

\( \ce{Ag+(aq) + Cl-(aq) -> AgCl(s)} \)
 

Other answers

→ Identify anion by testing pH with universal indicator.

→ Interpret result: Neutral (`text{Cl}^-` present), slightly basic (`text{CH}_3text{COO}^-` present), very basic (`text{OH}^-` present).


♦ Mean mark 49%.

CHEMISTRY, M8 2022 HSC 5 MC

Which pair of ions can be distinguished using a flame test in the school laboratory?

  1. `\text{Ag}^(+) and \text{Mg}^(2+)`
  2. `\text{Ba}^(2+) and \text{Ca}^(2+)`
  3. `\text{Br}^(-) and \text{Cl}^(-)`
  4. `\text{Fe}^(2+) and \text{Fe}^(3+)`
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`B`

Show Worked Solution

By elimination:

→ Silver and Magnesium do not emit visible wavelengths of light. (A is incorrect)}

→ The flame test only works on metals (C is incorrect)

→ Ions of the same element but different oxidation states cannot be distinguished using the flame test (D is incorrect)

→ In a flame test `text{Ba}^(2+)` has an apple-green flame colour whilst `text{Ca}^(2+)` has a brick-red flame colour, and thus can be distinguished. (B is correct)

`=> B`