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Transition metals (A-level only)

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Question 21

A student performs an experiment to determine the percentage of copper in a sample of brass.

The student:

  • reacts 850 mg850\text{ mg}850 mg of the brass with concentrated nitric acid to form a solution (all of the copper in the brass reacts to form aqueous copper(II) ions)
  • transfers the solution to a volumetric flask and makes the volume up to 250 cm3250\text{ cm}^3250 cm3 with distilled water
  • shakes the flask thoroughly
  • transfers 25.0 cm325.0\text{ cm}^325.0 cm3 of this solution to a conical flask and adds an excess of potassium iodide
  • titrates the mixture with 0.100 mol dm−30.100\text{ mol dm}^{-3}0.100 mol dm−3 sodium thiosulfate (Na2S2O3\text{Na}_2\text{S}_2\text{O}_3Na2​S2​O3​) solution, requiring exactly 12.00 cm312.00\text{ cm}^312.00 cm3 to react with all the iodine produced.

The equations for the reactions are:

2Cu2+(aq)+4I−(aq)→2CuI(s)+I2(aq) 2\text{Cu}^{2+}(aq) + 4\text{I}^-(aq) \rightarrow 2\text{CuI}(s) + \text{I}_2(aq) 2Cu2+(aq)+4I−(aq)→2CuI(s)+I2​(aq) 2S2O32−(aq)+I2(aq)→2I−(aq)+S4O62−(aq) 2\text{S}_2\text{O}_3^{2-}(aq) + \text{I}_2(aq) \rightarrow 2\text{I}^-(aq) + \text{S}_4\text{O}_6^{2-}(aq) 2S2​O32−​(aq)+I2​(aq)→2I−(aq)+S4​O62−​(aq)
1.

Calculate the percentage of copper by mass in the brass. Give your answer to the appropriate number of significant figures.

[5]
2.

Give the full electron configuration of a copper(II) ion.

[1]
3.

Copper(I) iodide is a white solid. Explain why copper(I) iodide is white whereas copper(II) compounds are typically coloured.

[3]
Markscheme

Transition metals (A-level only) Questions

  1. A Level
  2. /Chemistry
  3. /Transition metals (A-level only)

63 exam-style questions on AQA A Level Chemistry Transition metals (A-level only), covering General properties of transition metals (A-level only), Substitution reactions (A-level only), Shapes of complex ions (A-level only), Formation of coloured ions (A-level only), Variable oxidation states (A-level only), and Catalysts (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.

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