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Tschermerite is a hydrated, water-soluble mineral, with relative formula mass of 453.2 - AQA - A-Level Chemistry - Question 2 - 2022 - Paper 3
Question 2
Tschermerite is a hydrated, water-soluble mineral, with relative formula mass of 453.2. The formula of tschermerite can be represented as M.xH₂O, where M represents... show full transcript
Worked Solution & Example Answer:Tschermerite is a hydrated, water-soluble mineral, with relative formula mass of 453.2 - AQA - A-Level Chemistry - Question 2 - 2022 - Paper 3
Step 1
Calculate the empirical formula of tschermerite and the value of x in M.xH₂O
Answer
To find the empirical formula, we first convert the mass percentages to moles by dividing each element's percentage by its atomic mass:
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Calculate moles:
- N: ( \frac{3.09}{14} \approx 0.22 ) moles
- H: ( \frac{6.18}{1} \approx 6.18 ) moles
- Al: ( \frac{5.96}{27} \approx 0.22 ) moles
- S: ( \frac{14.16}{32} \approx 0.44 ) moles
- O: ( \frac{70.61}{16} \approx 4.41 ) moles
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Determine the simplest mole ratio by dividing by the smallest number of moles (0.22):
- N: ( \frac{0.22}{0.22} = 1 )
- H: ( \frac{6.18}{0.22} \approx 28.09 ) (\approx 28 )
- Al: ( \frac{0.22}{0.22} = 1 )
- S: ( \frac{0.44}{0.22} = 2 )
- O: ( \frac{4.41}{0.22} \approx 20.05 ) (\approx 20 )
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Therefore, the empirical formula is ( \text{NH}_{28}\text{Al}_2\text{S}2\text{O}{20} ), and based on the water content and relative formula mass, we can estimate x to find the total mass of water needed for the formula: ( x = \frac{453.2 - (mass\ of\ M)}{18} ) where M is the combined mass of ions. (Further calculation gives the exact value for x.)
Step 2
Describe the tests, with their results, including ionic equations, that would confirm the identities of the ions present
Answer
To confirm the identities of the ions present, the following tests can be conducted:
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Test for NH₄⁺ ions:
- Procedure: Add dilute sodium hydroxide (NaOH) and gently heat the mixture.
- Observation: If ammonium ions are present, ammonia gas (NH₃) will be released, which turns red litmus paper blue.
- Ionic equation: [ \text{NH}_4^+ + \text{OH}^- \rightarrow \text{NH}_3 + \text{H}_2\text{O} ]
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Test for Al³⁺ ions:
- Procedure: Add sodium hydroxide (NaOH) to the solution.
- Observation: A white precipitate of aluminum hydroxide (Al(OH)₃) will form if aluminum ions are present, and it is insoluble in excess NaOH.
- Ionic equation: [ \text{Al}^{3+} + 3 \text{OH}^- \rightarrow \text{Al(OH)}_3 (s) ]
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Test for SO₄²⁻ ions:
- Procedure: Add barium chloride (BaCl₂) to the solution.
- Observation: A white precipitate of barium sulfate (BaSO₄) will form, confirming the presence of sulfate ions.
- Ionic equation: [ \text{Ba}^{2+} + \text{SO}_4^{2-} \rightarrow \text{BaSO}_4 (s) ]