Gravimetric exercises (2010 VCE)

1) The amount of iron in a newly developed, heat-resistant aluminium alloy is to be determined.
An 80.50 g sample of alloy is dissolved in concentrated hydrochloric acid and the iron atoms are converted to Fe2+(aq) ions.
This solution is accurately transferred to a 250.0 mL volumetric flask and made up to the mark.
20.00 mL aliquots of this solution are then titrated against a standard 0.0400 M potassium permanganate solution.

5Fe2+(aq) + MnO4(aq) + 8H+(aq) → 5Fe3+(aq) + Mn2+(aq) + 4H2O(l)
Four titrations were carried out and the volumes of potassium permanganate solution used were recorded in the table below.

a. Write a balanced half-equation, including states, for the conversion of MnO4 ions, in an acidic solution, to Mn2+ ions. (2 marks)

b. Calculate the average volume, in mL, of the concordant titres of the potassium permanganate solution. (1 mark)

c. Use your answer to part b. to calculate the amount, in mol, of MnO4 (aq) ions used in this titration. (1 mark)
d. Calculate the amount, in mol, of Fe2+(aq) ions present in the 250.0 mL volumetric flask. (2 marks)
e. Calculate the percentage, by mass, of iron in the 80.50 g sample of alloy. Express your answer to the correct number of significant figures. (3 marks)

Click to see the assessor's report for question e.