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Homework Help: What am I doing wrong? chemistry problem

  1. Sep 15, 2011 #1
    1. The problem statement, all variables and given/known data
    If one leads chloride gas over salty water with a density of 1,1kg/L containing 4700ppm Bromide (Br-) then the bromide shall react with the chloride gas. How much chloride gas does one need to produce 1kg of Br2?

    2. Relevant equations
    The reaction is 2Br- + Cl2 --> Br2 + 2Cl-.. not really relevant, I think
    3. The attempt at a solution

    I think we can safely assume that the average molar mass of the water is ~ 19grams maybe , and thus the molar density is (1,1kg/19gramspermole)L = ~ 59 moles/L

    Br- concentration is 4,7 parts per thousand molecules. So concentration of Br- is 59*4,7*10^-3 moles/L = 0,2773moles/L.

    We needed 1Kg of Br2/Br-, with the molar mass of Br being 80g/mole. 1kg/80g/mole = 12,5 moles.

    12,5 moles/0,2773 moles= 45. We need 45L of salty water to produce 1Kg of Br2, in theory.

    But the correct answer is 192L.. What am I doing wrong?
    Last edited: Sep 15, 2011
  2. jcsd
  3. Sep 15, 2011 #2
    fk, i made quite the typo.. I said average density of the water is 19 grams when i meant average molar mass of the water molecules is roughly 19 grams.. facepalm

    anyway can somebody help me?
  4. Sep 15, 2011 #3


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    Staff: Mentor

    Actually that's the only relevant thing here. You are asked about amount of chloride, not of water. At least that's what you wrote.
  5. Sep 16, 2011 #4
    goddamn it, I meant how much *water* I need. it was late yesterday

    anyway apparantly 4700ppm can mean 4700 ppm of a kg, as well as 4700ppm of a mole. I didn't know that, so the problem is solved lol
  6. Sep 16, 2011 #5


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    Staff: Mentor

    Ppm can by anything - weight/weight, volume/volume, molecules/molecules (to name only those making sense). W/w is the most common one.
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