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iron tablets titration Watch

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    Attachment 616470Needing some help here please, in chemistry we have been determining the exact mass of iron in women's health iron tablets (Wilkos own). the bottle states each tablet contains 14mg of iron.

    Steps:
    crush up 5 iron tablets and dissolve in sulfuric acid, I believe the concentration and amount is not important, it is just to stop the iron from oxidising.

    make up a standard solution of 250cm^3
    ....
    KMnO4/H+ 0.02M

    when running through the following calculations it should titrate with 1.2cm^3 of Pottasium permangante solution (acidified) however the actual mean titre is 12cm^3, 10x more.

    5 x 14mg = 70mg
    (original mass - mass lost/original mass) x iron content = 67x10^-3 g
    67x10^-3/56(RMM) = 1.196x10^-3 mol
    1.196x10^-3 x 1000/250 = 4.786x10^-3 moldm^3 (iron solution)
    4.786x10^-3 x (25/1000) = 1.1965x10^-4 mol (used for one titration)
    (5 iron ions wirth one manganate ion) 1.1965x10^-4 / 5 = 2.393x10^-5 mols
    2.393x10^-5 / 0.02 = 1.1965x10^-3 dm^3 of KMnO4 solution
    dm^3 ---> cm^3 (x1000)
    1.1965x10^-3 x 1000 = 1.1965 cm^3

    Obviosuly wilkos arent going to bewrong on the mass of iron in each tablet (iron sulpahte i believe). so me the teacher and a friend in the class have ran through this calculation multiple times and still cannot find a mistake. the next logical thought was that the KMnO4/H+ solution was not actually 0.02M but 0.002M

    So we have now titrated the KMnO4 solution with a solution of iron sulpahte we made up our selves.
    Mr of iron sulphate = 278

    We made up a 0.1M solution of iron sulphate by dissolving+ 2.78g of iron sulphate into 100cm^3 solution.

    The mean titre was 25cm^3

    0.1 x (25/1000) = 2.5x10^-3 mol

    same as before 1:5 reaction

    2.5x10^-3 / 5 = 5x10^-4 mol in 25cm^3 therefore

    1000/25 x 5x10^-4 = 0.02M KMnO4 solution.

    I'm pretty much certain all the maths is correct (although not all the grammar and spelling is in this post) as 3 of us have checked it over multiple times. any suggestions to what could be going wrong?
    Thankyou

    EDIT: Just re-read this and some of the spelling is really bad but cant be bothered to correct it as i think its pretty obvious what i mean.
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    (Original post by bazvasey1)
    Needing some help here please, in chemistry we have been determining the exact mass of iron in women's health iron tablets (Wilkos own). the bottle states each tablet contains 14mg of iron.

    Steps:
    crush up 5 iron tablets and dissolve in sulfuric acid, I believe the concentration and amount is not important, it is just to stop the iron from oxidising.

    make up a standard solution of 250cm^3
    ....
    KMnO4/H+ 0.02M

    when running through the following calculations it should titrate with 1.2cm^3 of Pottasium permangante solution (acidified) however the actual mean titre is 12cm^3, 10x more.

    5 x 14mg = 70mg
    This is what you are trying to find out ...

    mol iron = 0.07/56 = 1.25 x 10-3 in 250ml solution

    Concentration = 5 x 10-3 mol dm-3

    How much of this standard solution did you titrate? 25ml ?

    If so moles of iron = 1.25 x 10-4

    This reacts with 2.5 x 10-5 mol manganate(VII) ions

    If the molarity of the manganate ions = 0.02M, this represents a volume of
    (2.5 x 10-5)/0.02 = 0.0125 litres = 12.5 ml




    (original mass - mass lost/original mass) x iron content = 67x10^-3 g
    67x10^-3/56(RMM) = 1.196x10^-3 mol
    1.196x10^-3 x 1000/250 = 4.786x10^-3 moldm^3 (iron solution)
    4.786x10^-3 x (25/1000) = 1.1965x10^-4 mol (used for one titration)
    (5 iron ions wirth one manganate ion) 1.1965x10^-4 / 5 = 2.393x10^-5 mols
    2.393x10^-5 / 0.02 = 1.1965x10^-3 dm^3 of KMnO4 solution
    dm^3 ---> cm^3 (x1000)
    1.1965x10^-3 x 1000 = 1.1965 cm^3

    Obviosuly wilkos arent going to bewrong on the mass of iron in each tablet (iron sulpahte i believe). so me the teacher and a friend in the class have ran through this calculation multiple times and still cannot find a mistake. the next logical thought was that the KMnO4/H+ solution was not actually 0.02M but 0.002M

    So we have now titrated the KMnO4 solution with a solution of iron sulpahte we made up our selves.
    Mr of iron sulphate = 278

    We made up a 0.1M solution of iron sulphate by dissolving+ 2.78g of iron sulphate into 100cm^3 solution.

    The mean titre was 25cm^3

    0.1 x (25/1000) = 2.5x10^-3 mol

    same as before 1:5 reaction

    2.5x10^-3 / 5 = 5x10^-4 mol in 25cm^3 therefore

    1000/25 x 5x10^-4 = 0.02M KMnO4 solution.

    I'm pretty much certain all the maths is correct (although not all the grammar and spelling is in this post) as 3 of us have checked it over multiple times. any suggestions to what could be going wrong?
    Thankyou

    EDIT: Just re-read this and some of the spelling is really bad but cant be bothered to correct it as i think its pretty obvious what i mean.
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    (Original post by charco)
    This is what you are trying to find out ...

    mol iron = 0.07/56 = 1.25 x 10-3 in 250ml solution

    Concentration = 5 x 10-3 mol dm-3

    How much of this standard solution did you titrate? 25ml ?

    If so moles of iron = 1.25 x 10-4

    This reacts with 2.5 x 10-5 mol manganate(VII) ions

    If the molarity of the manganate ions = 0.02M, this represents a volume of
    (2.5 x 10-5)/0.02 = 0.0125 litres = 12.5 ml
    It's all good until where (2.5x10^-5/0.02) = 1.25x10^-3, which is in dm3? So x1000 to get to cm3 which is 1.25 cm3 or ml? Correct me if I'm wrong
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    (Original post by bazvasey1)
    It's all good until where (2.5x10^-5/0.02) = 1.25x10^-3, which is in dm3? So x1000 to get to cm3 which is 1.25 cm3 or ml? Correct me if I'm wrong
    yes, you are correct. Poor mental arithmetic on my part...
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    (Original post by charco)
    yes, you are correct. Poor mental arithmetic on my part...
    No worries we all do it. Wonder why it still don't make sense?
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    (Original post by bazvasey1)
    No worries we all do it. Wonder why it still don't make sense?
    Your maths all looks okay. In the calculations it suggests you used one tenth of the iron solution and this should take 1.2cm^3

    Is it possible that you didn't in fact use 1/10th but instead used the whole solution?

    This would explain the factor of 10 in the answer?

    Just trying to work out whats going on!
    :smile:
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    (Original post by MexicanKeith)
    Your maths all looks okay. In the calculations it suggests you used one tenth of the iron solution and this should take 1.2cm^3

    Is it possible that you didn't in fact use 1/10th but instead used the whole solution?

    This would explain the factor of 10 in the answer?

    Just trying to work out whats going on!
    :smile:
    Nope definitely used 1/10th, 25ml pipette from a volumetric flask😊
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    (Original post by bazvasey1)
    Nope definitely used 1/10th, 25ml pipette from a volumetric flask😊
    fair enough, it was just a thought! :P
 
 
 
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