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    A problem from Jan ’04 Polymers Proteins and Steel (now Chemistry of Materials)

    Right: Q4a) This is a long one, so here goes

    Gardeners use ‘lawsand’ to kill moss and improve the appearance of their lawns. Lawsand contains a mixtureof iron(II) sulphate and sand.
    A group of students analysed the percentage of iron (II) sulphate, FeSO4 in a sample of lawsand.
    They found that adding water to lawsand dissolved the iron(II) sulphate.
    They decided to use a redox titration with aqueous potassium manganate (VII) to find the concentration of Fe2+ ions in the solution.
    Outline the procedure the students could use for this analysis, starting with a weighed sample of lawsand and a standard solution of aqueous potassium manganate(VII).

    6 marks. I have got in a mess with my notes - how exactly would you do this for the 6 marks?

    Any help much appreciated.
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    I was just about to type out the exact same thread.

    I'm looking through SS2.2 to try and come up with an answer for it
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    Okay, here's a sketchy answer. dissolved weighed sample of lawnsand in distilled water (say what volume?)
    set up and fill a burette with standars solution of potass mang
    use pipette, extract 10cm3 (or any amount) of lawnsand solution, place in conical flask and titrate. Titrate until the first pink colour appears that lasts for over 30seconds
    Record volume of titrant added, and use balanced equation to work out the concentration of Fe2+ ions.

    I was going to add in there something about repeats?
    I also do not see how you can get six marks?

    edit : perhaps marks for remembering or deducing an equation with manganate and Fe ions?
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    (Original post by kohlstream)
    Okay, here's a sketchy answer. dissolved weighed sample of lawnsand in distilled water (say what volume?)
    set up and fill a burette with standars solution of potass mang
    use pipette, extract 10cm3 (or any amount) of lawnsand solution, place in conical flask and titrate. Titrate until the first pink colour appears that lasts for over 30seconds
    Record volume of titrant added, and use balanced equation to work out the concentration of Fe2+ ions.

    I was going to add in there something about repeats?
    I also do not see how you can get six marks?

    edit : perhaps marks for remembering or deducing an equation with manganate and Fe ions?
    Sounds good. In my notes I've got "Add H2SO4". I figured I'd remember what i was on about...but i don't. What d'you think that refers to? I can't see where some sulphuric acid would be helpful. Also, i suspect you're right about the mark for repeats, perhaps rough titration to begin with. I doubt there's a mark for any equation. Otherwise, that seems like 5 or 6 marks.

    In my notes as well, i've got "filter the lawsand". Obviously i thought it was right when i wrote it down, but I can't that there's a mark for it - u?

    Another thing - i think there's probably a mark for diluting the solution with the lawsand in it because the concentration is likely to be quite high.

    Edit: Just thought of another possible mark - put the conical flask on a white tile?
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    (Original post by CrazyChemist)
    Sounds good. In my notes I've got "Add H2SO4". I figured I'd remember what i was on about...but i don't. What d'you think that refers to? I can't see where some sulphuric acid would be helpful. Also, i suspect you're right about the mark for repeats, perhaps rough titration to begin with. I doubt there's a mark for any equation. Otherwise, that seems like 5 or 6 marks.

    In my notes as well, i've got "filter the lawsand". Obviously i thought it was right when i wrote it down, but I can't that there's a mark for it - u?

    Another thing - i think there's probably a mark for diluting the solution with the lawsand in it because the concentration is likely to be quite high.

    Edit: Just thought of another possible mark - put the conical flask on a white tile?
    in the activity, i *think* you add H2SO4 for something to do with preparation of the spinach leaves
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    (Original post by kohlstream)
    in the activity, i *think* you add H2SO4 for something to do with preparation of the spinach leaves
    This wasn't a note from the activity - the question paper was set as homework and gone through in class, but the teacher was going through things pretty quickly so i just made lazy bullet points. It would seem that the H2SO4 was something to do with this question.
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    (Original post by CrazyChemist)
    This wasn't a note from the activity - the question paper was set as homework and gone through in class, but the teacher was going through things pretty quickly so i just made lazy bullet points. It would seem that the H2SO4 was something to do with this question.
    feck.

    would it act as an oxidising agent to make sure the FE is Fe2+?
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    I've got the markscheme for this paper so.. i thuoght you guys would like to know.. (unless someone else has already put it up)

    -Make up to known volume with water.
    -Add acid (must be H2SO4 if named)

    Alternative: Make up to known volume with dilute acid

    - until first permanent pink colour - not with indicator


    then 3 from:
    - Filter solution
    - Wash with water
    - Dilute stock solution
    - Pipette sample
    - Add MnO4- from burette
    - repeat

    Hope that helps!
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    THANK YOU!!!!!!!!!

    I really don't understand why they add acid, but there ya go!
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    Thanks for that! What's the acid for? I know I should know this , but what on Earth is it for???
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    to react with the MnO4-?! Thats all i can think of right now..

    is the product from titrating an alkali?
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    MnO4- is purple...it goes clear when it reacts with the Fe2+...The solution is slightly pink when there's no more Fe2+ to react with. That's my understanding of the titration...no idea where acid or alkali comes into it...
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    do you knwo what unit this would come under?
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    (Original post by roochis)
    do you knwo what unit this would come under?
    Activity SS1.2

    On the previous page, it mentions that sulphuric acvid is used to dissolve potassium manganate (VII )...?
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    Its a redox reaction apparently.. is it there to donate protons?.. no wait.. that would make it an acid base reaction.. ignore me.. i'll keep searching!
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    (i)MnO4-(aq) + 8H+(aq) + 5e-=> Mn2+(aq) + 4H2O(l)

    (ii) Fe3+(aq) + e- => Fe2+(aq)

    The wonders of the internet! this is what it gave me.. so the acid needs to donate protons i guess!
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    (Original post by roochis)
    Its a redox reaction apparently.. is it there to donate protons?.. no wait.. that would make it an acid base reaction.. ignore me.. i'll keep searching!
    i thought it may besomething with donating protons to the fe to make sure it was fe2+, but it's prolly not that

    this will bug me all night!
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    (Original post by roochis)
    do you knwo what unit this would come under?
    Nope - we just have to know it for this exam (at least). That's all i know.
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    its steel story.. just incase you wanted to know!

    if you type doc brown salters into google you should come up with a site which is quite helpful for chem salters course!
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    do you reckon we need to know that equation?
 
 
 
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