Chemical amounts (A2)

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Hi. I am using a technique called a “back” titration in order to determine the amount of iron(II) in my iron tablets. I was advised to titrate the iron(II) (iron(II) is already mixed with sulphuric acid) with excess potassium manganate(VII), and then to react the left over potassium manganate(VII) with hydrogen peroxide. How would I determine the amount of hydrogen peroxide to react with the excess potassium manganate(VII) and the concentration of iron(II)?

Igloo writes
You have adopted a rather unusual method, since this determination is normally carried out with a direct titration of the solution with potassium manganate(VII), without the need for a back titration. In any case, the outline steps are as follows.
 
1 Use the titration reading and the known concentration of the H₂O₂ to determine the number of moles of this solution added from the burette.
 
2 From the equation between H₂O₂ and KMnO₄, calculate the number of moles of KMnO₄ left in excess.
 
3 By subtraction from the original number of moles of KMnO₄ added, you can deduce the number of moles of KMnO₄ which reacted with the iron(II) ions.
 
4 From the equation for the reaction between iron(II) ions and manganate(VII) ions, calculate the number of moles of iron(II) ions present in the solution.
 
5 Assuming that a pipette was used to transfer a known volume of the iron(II) solution for this experiment, scale up to find the number of moles of iron(II) ions originally weighed out.
 
6 Convert this to a mass, via the atomic mass of iron, and, together with the mass of tablets weighed out, you can calculate the percentage of iron in the tablets.
 
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updated: 17 December 2006