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Math Help - Help with this chemistry question!

  1. #1
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    Help with this chemistry question!

    An iron nail was dissolved in excess dilute sulphuric acid to produce 100 cm3 solution. A sample of 10 cm3 of this solution required 4 x 10-4 mol of potassium manganate (VII) for complete reaction.


    Fe(s) + H2SO4 (aq) Fe2SO4 (aq) + H2 (g)
    5Fe2+ (aq) + MnO4- Mn2+ + 4H2O (l) + 5Fe3+ (aq)

    If the mass of nail used was 1.45g, calculate the number of moles of Fe2+ produced in the reaction.

    The solution they gave was simple:

    5 x (4 x 10-4) x 10
    = 0.02 mol.

    But I don't understand. If 1 mol of MnO4 requires 5 mol of Fe2+, wouldn't it mean that 1 mol of Fe 2+ uses 1/5 mol of MnO4 to produce 1 mol of Fe3+? So, shouldn't it be 4 x 10-4 / 5 instead of 4 x 10-4 x 5?

    Also, why do they need to x 10?

    My solution was 1.45 / 56 = 0.03 mol since 1 mol of Fe require 1 mol of H2SO4 to produce 1 mol of FeSO4. Why is this not accepted? Thanks.
    Last edited by mark1950; October 11th 2009 at 06:39 AM.
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  2. #2
    Junior Member
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    Aug 2009
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    You multiply by 10 cause you titrated 10cms only of the slution... when there is 100cm

    And the logic up there is very simple, 1 mol of km04 reacts with 5Fe2 ? Right, its not asking you how much of km04 1 mol of fe2+ require but rather the amount of Fe2+ used, so if 1 mol of kmo4 was used, it had to react with 5 mole of fe2+
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