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gChem Dat Destroyer Q [need book to answer]

CANgnome

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Mar 16, 2010
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  1. Dental Student
    Anyone an explain to me number 116 and 108?


    116)
    why is normality 3 instead of 6, since a total of 6 electrons are transferred from dichromate. I understand that it goes from 6+ chromium to 3+ chromium. and it is 3x e transfer, but the reaction involves 2 chromium atoms. shouldn't it be 6?

    108)
    i just don't get it. i understand you have to normality the Ca(OH)2, but how can the concentration of Ca(OH)2 be used to calculate the concentration of HBr using N1V1=N2V2? I assume they will react somehow? Or are they missing some part of the explanation.
     

    UndergradGuy7

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    Jun 24, 2007
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    1. Pre-Dental
      Anyone an explain to me number 116 and 108?


      116)
      why is normality 3 instead of 6, since a total of 6 electrons are transferred from dichromate. I understand that it goes from 6+ chromium to 3+ chromium. and it is 3x e transfer, but the reaction involves 2 chromium atoms. shouldn't it be 6?

      108)
      i just don't get it. i understand you have to normality the Ca(OH)2, but how can the concentration of Ca(OH)2 be used to calculate the concentration of HBr using N1V1=N2V2? I assume they will react somehow? Or are they missing some part of the explanation.

      Different book versions, have different numbers. Post the question for faster responses.

      I think I found the ones you are talking about. Ca(OH)2 gives of 2 OH-... OH- is a base and reacts with H+ from HBr. So if you know the moles of OH-, you know the moles of H+. moles H+/ L HBr = molarity of HBr

      so 0.024N*20mL gives you millimoles of OH-. This equals millimoles of HBr.
       
      Last edited:

      UndergradGuy7

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        The other question was...
        Consider 2.5 M dichromate:
        Cr2O7^2- -> Cr^3+ (acidic medium)

        What would be the solution Normality if the reaction goes to completion?

        I thought the answer was 15 N. Since if you balance it there is 6e-. Cr in Cr2O7^2- is +6 and it goes to +3 in Cr^3+.

        So 6e-*2.5=15N

        but destroyer says 7.5N. I am not entirely sure if destroyer is right either? I don't know why he says you don't need to balance it.

        If you don't balance it and assume that the book just wants 1 Cr^3+ then only 3e- are being used. Then the normality is 7.5N.

        But he still says in the answer key that it does not matter if you balance it or not. But it goes matter?
         

        CANgnome

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          you are right on the button for what i was thinking for question 108 as well. I am thinking that the book should have at least mentioned something about it reacting, seems rather vague.

          anyways, for 116: very confused still. balance or not to balance. he obvoiously means 3e-'s because his final answer is 7.5, but what is the principle behind not balancing? That is the question.
           
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