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Question on inorganic chemistry

  1. Jul 19, 2009 #1
    1. The problem statement, all variables and given/known data

    You are supplied with a mixture consisting of barium carbonate,dolomite,potassium carbonate and silicon dioxide.Explain how you would attempt to determine quantitatively each of the constituents present in this mixture

    2. Relevant equations



    3. The attempt at a solution
    First I Weigh the mixture.

    Then,since potassium carbonate would dissolve in water as all group 1 compounds are soluble, I first add water,and filter.The filtrate contains dissolved potassium carbonate,which I dry,weigh and calculate its % composition
    To the residue I add HCl,so the SiO2 remains which i later dry and weigh?
    and in the filtrate I get CaCl2,MgCl2and BaCl2.
    I don't know how to proceed from here.I'm tempted to add NaOH,but then all 3 i.e,Ca(OH)2,Ba(OH)2 and Mg(OH)2 would precipitate,and I wouldn't be able to seperate them?

    Thank you
     
  2. jcsd
  3. Jul 20, 2009 #2

    Lok

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    If wiki is right then dolomite means CaMg(CO3)2 that means echimolar conposition of the two. By weighing the rest after removal of Sio and KCO3 than a simple 2 unknowns equation will suffice.
     
  4. Jul 20, 2009 #3
    I knew that dolomite was made of CaCO3 and MgCO3 but I didn't know they were each equimolar in composition,so thank you very much for that useful info.
    Now I think I can manage to solve this problem easily if only I knew how to remove BaCO3 from the mixture,without affecting the composition of dolomite.

    I hope someone can help.
     
  5. Jul 20, 2009 #4

    Lok

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    Is it necessary to remove each compound from the mixture or just to determine it's composition. Because in the case of the composition you already made enough transformations. You have the weight and the mole number of the CaCl2, MgCl2 and BaCl2 as you know how much HCl you reacted in the SiO2 separation.
     
  6. Jul 20, 2009 #5

    chemisttree

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    http://www.mineralszone.com/minerals/dolomite.html" [Broken] on a forum!

    It occurs to me that you should research the solubilities of barium salts... and don't limit yourself to the chlorides.
     
    Last edited by a moderator: May 4, 2017
  7. Jul 21, 2009 #6

    Lok

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    Don't know where you are going with this as the mass % in the site still point to echimolarity of dolomite. Further separation is still not necessary.
     
    Last edited by a moderator: May 4, 2017
  8. Jul 21, 2009 #7

    Borek

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    Staff: Mentor

    It doesn't. Equimolar is only an ideal case, real life samples don't have to be ideal.

    I am afraid question is unfortunate, as it is not necesarilly clear what to make of the 'dolomite' information - it means either 'equimolar mixture of Mg/Ca', or 'some mixture of Mg/Ca'.
     
  9. Jul 21, 2009 #8

    Lok

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    If we question the purity of every compound ... then what is the point of it. Every step of the analysis will have it's tolerances.
    Unless mass spectrometry or other means are present.
     
  10. Jul 21, 2009 #9

    Borek

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    Staff: Mentor

    Dolomite is not a compound, dolomite is a mineral. Barium carbonate is a compound and nobody questions its purity. Minerals are rarely pure compounds.
     
  11. Jul 21, 2009 #10

    Lok

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    True to these words. This means that all reactions till now have to account for dolomite's impurities.

    There is a limit to this so called helping process ... But I'll get you a beer someday Borek.
     
  12. Jul 21, 2009 #11
    I don't really know what I should do really,cause

    I've attempted another question related to dolomite(which i'm hoping i could solve after completiing this).In that question,it states"A large quantity of dolomite containing silica as the only impurity is provided" ,so I have a feeling they would have mentioned it if the sample was impure,but I'm not very sure

    But I also feel that it may not be very accurate to take dolomite as an equimolar mixture of CaCO3 and MgCO3,

    so I think its best to avoid the equimolar part and also to ignore any impurities found in dolomite cause afterall we're only asked to find quantitatively each of the constituents present(I could be wrong but we're not really asked to find the composition of MgCO3 and CaCO3 seperately right?i don't know if this is even possible but wouldn't it be just enough to find the mass of dolomite as a whole?or is this what you all have been saying so far?I can't really makeout what's going on :( )


    I researched and i don't know for sure but here's what I think,
    To the chloride filtrate I obtained in my first post I add excess sulphuric acid.Then I get a precipitate of BaSO4 which I filter ,dry and weigh and calculate the no. of moles which is equal to that of barium carbonate and in the filtrate I have MgCl2/CaCl2 ?
    And since I now have the weights of potassium carbonate,SiO2 and BaCO3,I can substract their weights from the initial weight of the mixture to find the weight of dolomite,perhaps?
     
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