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The clock glasses shown in the diagram contained pure samples of the following salts: KCl, KNO₃, Na₂HPO₄·12H₂O, Na₂SO₄·7H₂O, NaHCO₃, and Na₂SO₄·10H₂O - Leaving Cert Chemistry - Question 3 - 2009
Question 3
The clock glasses shown in the diagram contained pure samples of the following salts: KCl, KNO₃, Na₂HPO₄·12H₂O, Na₂SO₄·7H₂O, NaHCO₃, and Na₂SO₄·10H₂O. Each clock gla... show full transcript
Worked Solution & Example Answer:The clock glasses shown in the diagram contained pure samples of the following salts: KCl, KNO₃, Na₂HPO₄·12H₂O, Na₂SO₄·7H₂O, NaHCO₃, and Na₂SO₄·10H₂O - Leaving Cert Chemistry - Question 3 - 2009
Step 1
Describe how the student could have distinguished between the samples that contained potassium ions and those that contained sodium ions using the flame test technique.
Answer
To distinguish between potassium and sodium ions using the flame test, the student should follow these steps:
- Introduce a small amount of each salt sample into the flame of a Bunsen burner using a platinum or nichrome wire that has been cleaned thoroughly.
- Observe the color of the flame produced:
- Sodium ions (Na⁺) will impart a bright yellow color to the flame.
- Potassium ions (K⁺) will produce a lilac or light purple flame.
- Should a colored flame be observed, compare the color to determine the presence of potassium or sodium ions, with sodium showing a strong yellow flame, while potassium is characterized by its light purple hue.
Step 2
One of the samples above was identified by the addition of silver nitrate, AgNO₃, solution to a solution of each sample in turn? What observation indicated a positive test result?
Answer
When a few drops of silver nitrate (AgNO₃) solution are added to each of the salt solutions, the formation of a white precipitate indicates the presence of halides. Specifically, the formation of a white precipitate of silver chloride (AgCl) will confirm the presence of chloride ions (from KCl). The positive test result is signified by the appearance of this white precipitate.
Step 3
Describe how you would test the samples for the presence of the phosphate anion.
Answer
To test for the presence of phosphate ions (PO₄³⁻), follow these steps:
- Take a small sample of the suspected phosphate salt and dissolve it in a few milliliters of distilled water.
- Add a few drops of ammonium molybdate solution, and then add concentrated nitric acid (HNO₃) to the mixture gently.
- Heat the solution gently. If a yellow precipitate forms, this indicates a positive test for phosphate ions, as ammonium molybdate reacts with phosphate to form a yellow ammonium phosphomolybdate complex.
Step 4
Having completed the tests referred to in (a) - (c) above, the student should have positively identified the six salts. Describe how you would conduct a systematic test of each salt to confirm the identity of each salt by a process of elimination.
Answer
To systematically identify each salt, the following process can be employed:
- Initial Observation: Record the physical appearance of each salt (color, texture, etc.).
- Flame Tests: Conduct the flame test as described in part (a) to identify potassium and sodium salts.
- Silver Nitrate Test: Add silver nitrate to each salt solution to identify chloride salts (KCl) via precipitate formation.
- Phosphate Test: As described in part (c), test for the presence of phosphate ions to identify Na₂HPO₄·12H₂O.
- Reactions with Acids: For the remaining salts, react with dilute hydrochloric acid (HCl) to observe if any gas evolved (CO₂ for NaHCO₃). The result will help narrow down the possibilities.
- Final Confirmation: Compare observations, notes, and results against the initial salt list to confirm identities by elimination, ensuring the rationale for identifications is documented well.