Determination of iron purity by redox reaction

[izzypink]

Hi just wondering if you could help, I've stumbled across an example question where the iron purity is meant to be above 80% when

8H+ + 5Fe2+ + MnO4- → 5Fe3+ + Mn2+ + 4H2O

15cm3 is titrated and concentration of potassium permanganate used is 0.02M

So far I have worked out

Number of moles of potassium permanganate=

Concentration x Volume(cm3) = 0.02 x 15 = 3x10-4 moles
1000 1000


From the equation the ratio of moles of potassium permanganate to moles of iron is 1:5

Therefore 3x10-4 x 5= 1.5x10-3 Moles of Iron for sample “A”


Mass of Iron: Molesx Relative atomic mass= 1.5x10-3 x 55.845 = 0.0837675g

The original mass of sample is 3.80g and the iron purity in the answers is around 88-89%

Can anybody help me understand where I've gone wrong?

 

[Borek]

Sounds to me like 15 mL is volume of titrated sample, not volume of the titrant. I guess there are other details - like dilution of the sample - that can be important.

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