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2 marksOxidation – Reduction Reactions

CSEC Chemistry · January 2019 · Paper 2 · Question 1(a)

An experiment was conducted to determine the percentage of iron in an iron salt sample. A 0.500 g sample of iron salt was placed in a conical flask, to which 25.0 mL of dilute sulfuric acid, 10 mL of phosphoric acid, and 8 drops of an indicator were added. The mixture was titrated against a 0.020 mol dm⁻³ solution of potassium dichromate until the endpoint. The ionic equation for the reaction is Cr₂O₃²⁻ + 6Fe²⁺ + 14H⁺ → 6Fe³⁺ + 2Cr³⁺ + 7H₂O. Figure 1 shows the initial and final burette readings for each titration.

List the apparatus that would be necessary to carry out the experiment.

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Other parts of this question

  1. 1(b)Use the information in Figure 1 to complete Table 1.[9 marks]
  2. 1(c)(i)Use an asterisk (*) to indicate the titration data that should be used to obtain the average titration volume of potassium dichromate used in the experiment.[1 mark]
  3. 1(c)(ii)Hence, calculate the average volume of potassium dichromate used in the experiment and record the value to two decimal places.[2 marks]
  4. 1(d)Using the information from (c)(ii), calculate the average number of moles of potassium dichromate used in the experiment.[1 mark]
  5. 1(e)From the equation given on page 4, determine the number of moles of iron ions (Fe²⁺) that react with 1 mole of dichromate ions (Cr₂O₇²⁻).[1 mark]
  6. 1(f)Calculate the number of moles of iron in the iron salt sample.[1 mark]
  7. 1(g)Calculate the mass of iron in the iron salt sample. [Molar mass of Fe is 55.8 g mol⁻¹.][1 mark]
  8. 1(h)Calculate the percentage of iron in the iron salt sample.[1 mark]
  9. 1(i)The reaction between iron and dichromate is considered a reduction-oxidation reaction. Define the term 'reduction' in terms of electrons.[1 mark]
  10. 1(j)In the reaction, Fe²⁺ acts as a reducing agent. Define the term 'reducing agent'.[2 marks]
  11. 1(k)Calculate the oxidation state of Cr in Cr₂O₇²⁻.[2 marks]
  12. 1(l)State ONE precaution that should be taken when carrying out the experiment.[1 mark]

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