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Homework Help: Molar ratio

  1. Mar 28, 2016 #1
    1. The problem statement, all variables and given/known data
    Hi everyone, i have a problem. My brain is short-circuited, and it seems like i can't find out the simplest chemistry in the world :D

    We made this experiment where we should determine the sugar content in potato flour. We added amylases to break down the starch to glucose. Then we added fehling's solution and isolated the sediments. Then we added some ammoniumiron(III)sulphate. Last we titrated the solution with potassiumpermanganate.

    My problem is that i need to find the molar ratio between the sugar/glucose and the permanganate-ion.

    2. Relevant equations
    The chemical reactions is given by the following reactions.

    1. 5OH- + RCHO + 2Cu2+ --> RCOO- + Cu2O + 3H2O
    2. 2H+ + Cu2O + 2Fe3+ --> 2Cu2+ + 2Fe2+ + 2H2O
    3. 8H+ + 5Fe2+ + MnO4- --> 5Fe3+ + Mn2+ + 4H2O
    3. The attempt at a solution
    The molar ratio between RCHO and Cu2O is 1:1. So is the ratio between Cu2O in equation 1 and 2.
    The molar ration between Cu2O and Fe2+ is 1:2. So the ratio between RCHO and Fe2+ is also 1:2. But the ratio between Fe2+ in equation 2 and 3 is 2:5. So the ratio between Fe2+ and RCHO is 1:(2*2,5) = 1:5? Because there for every Fe(II) in equation 2 there is 2,5 Fe(II) in equation 3.

    But the then the ratio between Fe2+ and MnO4- in equation 3 is 5:1.
    So the ratio between MnO4- and RCHO is what? 5:5? That cannot be true?

    My logic says 1:2,5. But i can't explain why. It maked sense to me at first, but when i tried to explain why, i couldn't. Could you help me figure out what the real molar ratio is, and how to explain why it is that? Sorry, that i suddently can't figure out the simplest chemistry and math in the world :D
  2. jcsd
  3. Mar 28, 2016 #2
    Well i just tried working it out entering numbers. Which i think makes more sense to me.

    If i have 1 mol RCHO, i have 1 mol Cu2O.
    Then i have 2 mols Fe2+
    In equation 3 i still have 2 mol Fe(2+, which means i have 2/5 mol MnO4-.

    Then the molar ratio between RCHO and MnO4- must be 1:0,4. Which means for every mol of MnO4- i have 2,5 mol RCHO.
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