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1 markMole Concept

CSEC Chemistry · January 2017 · Paper 2 · Question 1(a)

An experiment was conducted to determine the number of moles of water of crystallization, n, in hydrated iron(II) sulfate, FeSO4•nH₂O. A sample was analyzed by titrating its iron(II) sulfate content with a standard solution of potassium manganate(VII), KMnO4.

Define the term 'standard solution'.

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  1. 1(b)Complete Table 1 by calculating the mass of the hydrated iron(II) sulfate used.[1 mark]
  2. 1(c)(i)Record the final burette volumes from the diagrams in Figure 1 in the appropriate spaces in Table 2.[3 marks]
  3. 1(c)(ii)Calculate the volume of KMnO4 solution used in EACH titration and enter them in Table 2.[3 marks]
  4. 1(c)(iii)Determine the average volume of KMnO4 solution used in the titrations.[1 mark]
  5. 1(d)(iv)Calculate the number of moles of KMnO4 in the average volume determined in (c)(iii).[1 mark]
  6. 1(d)(i)As shown in the equation, 1 mole MnO4⁻ reacts with 5 moles of Fe²⁺. Using the result in (c)(iv), calculate the number of moles of Fe²⁺ ions in the 25.0 cm³…[1 mark]
  7. 1(d)(ii)Determine the number of moles of Fe²⁺ in the 250.0 cm³ volumetric flask.[1 mark]
  8. 1(e)Given that 1 mole of FeSO4 contains 1 mole of Fe²⁺ ions, use the result from (d)(ii) to calculate the mass of anhydrous FeSO4 in the 250 cm³ volumetric flask.…[1 mark]
  9. 1(f)Calculate the mass of water in the hydrated FeSO4 using the following formula: Mass of water = mass of hydrated FeSO4 [from (b)] - mass of anhydrous FeSO4…[1 mark]
  10. 1(g)Calculate the number of moles of water in the hydrated sample. [The relative molecular mass of water is 18.0.][1 mark]
  11. 1(h)Using the results from (d)(ii) and (g), calculate the value of n in the formula FeSO4•nH₂O. n = number of moles of water in hydrated sample / number of moles…[1 mark]
  12. 1(i)What is the colour change at the endpoint of the titration of iron(II) sulfate with potassium manganate(VII)?[2 marks]
  13. 1(j)State ONE reason why there was no need to add an indicator to this titration.[1 mark]
  14. 1(k)(i)Aqueous sodium hydroxide was added dropwise, and then in excess. Inference: Fe²⁺ ions present.[2 marks]
  15. 1(k)(ii)The resulting mixture from (i) was left to stand in air. Inference: Fe²⁺ ions oxidized to Fe³⁺.[2 marks]
  16. 1(k)(iii)Aqueous barium nitrate was added, followed by dilute nitric acid. Inference: SO4²⁻ ions present.[2 marks]

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