1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    Borek

    User Avatar

    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

    Borek

    User Avatar

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook