[Paulo1913]

 This is a practice question and I am stuck on how to answer it... please could someone help me?


(a)   Glycerine, a mixture of glycerol and water, weighing 100.0 mg was treated with 50.0 mL of 0.0837 mol L–1 Ce4+ solution in 4 mol L–1 HCl at 60°C for 15 minutes. The excess Ce4+ solution needed 12.11 mL of 0.0448 mol L–1 iron(II) ammonium sulfate to reach the end point.

   The unbalanced equations are:

   HOCH2CH(OH)CH2OH   →   HCOOH
      (glycerol)   (methanoic acid)

   Ce4+   →   Ce2+

   Fe2+   →   Fe3+

   (i)   Calculate the percentage glycerol in the glycerine sample.
       M(glycerol) = 92.0 g mol–1

   (ii)   Discuss why a back titration is used and compare the errors introduced in a back titration with a direct titration.

(b)   The lines on the diagram below show the variation in reduction potential of water with acidity for the following reactions.
   Line {a}: O2(g)  +  4H+(aq)  +  4e–  →  2H2O(ℓ)
   Line {b}: 2H2O(ℓ)  +  2e–    →   H2(g)  +  2OH–(aq)


   Use the information from this diagram to answer the question below.

   Discuss how the vanadium species present in aqueous solution (saturated with oxygen) changes as the pH is raised from 0 to 14. 
         
      VO2+   +   2H+(aq)   +   e–     →  VO2+(aq)  +  H2O      E° =   + 1.00 V   
      VO2+(aq)  +  2H+(aq)  +  e–  →   V3+(aq)  +  H2O      E° =   + 0.34 V