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1 markThe Mole Concept

CAPE Chemistry Unit 1 · May/June 2024 · Paper 2 · Question 1(f)(ii)

Potassium nitrate decomposes on heating: 2 KNO3(s) -> 2 KNO2(s) + O2(g). A sample of KNO3 weighing 6.20 g (Sample A) was partially decomposed. The residue was dissolved in water to make 1 dm^3 of solution. A 25.0 cm^3 aliquot was acidified and titrated with 0.02 mol dm^-3 KMnO4, requiring an average titre of 27.50 cm^3.

Using the equation 2 MnO4^-(aq) + 5 NO2^-(aq) + 6 H^+(aq) -> 2 Mn^2+(aq) + 5 NO3^-(aq) + 3 H2O(l), deduce the number of moles of KNO2 present in the titre volume.

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Other parts of this question

  1. 1(a)Name the series of lines produced by these electron transitions.[1 mark]
  2. 1(b)Draw diagrams (including the axes) to show the shapes of the s and p orbitals.[2 marks]
  3. 1(c)State what is meant by the term 'hybridization'.[2 marks]
  4. 1(d)Benzene exhibits resonance structures. Explain what is meant by 'resonance'.[2 marks]
  5. 1(e)(i)State THREE criteria used in the valence shell electron pair repulsion (VSEPR) theory to determine the shapes of molecules and ions.[3 marks]
  6. 1(e)(ii)a)Use the criteria in (e)(i) to determine the shape and bond angles in the beryllium chloride molecule.[2 marks]
  7. 1(e)(ii)b)Use the criteria in (e)(i) to determine the shape and bond angles in the methyl anion, CH3^-.[2 marks]
  8. 1(e)(iii)Draw a diagram to show the shape of the methyl anion, CH3^-.[1 mark]
  9. 1(f)(i)Calculate the number of moles of potassium manganate(VII) used in the titration.[1 mark]
  10. 1(f)(iii)Calculate the number of moles of KNO2 that was present in the residue from the decomposition of Sample A.[1 mark]
  11. 1(f)(iv)Calculate the mass of KNO2 present in the residue from Sample A.[2 marks]
  12. 1(f)(v)Using the equation for the decomposition of KNO3, determine the mass of KNO2 formed if 6.20 g KNO3 were completely decomposed.[3 marks]
  13. 1(f)(vi)Using the results from (f)(iv) and (v), calculate the percentage of KNO2 formed.[2 marks]
  14. 1(f)(vii)Outline the experimental steps taken in the laboratory to prepare the 0.02 mol dm^-3 potassium manganate(VII) solution used for this titration.[5 marks]

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