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Tough Problem on Chemical Equilibrium

  1. Sep 17, 2010 #1
    1. The problem statement, all variables and given/known data

    A saturated solution of iodine in water contains 0.330g of I2/L. More than this can dissolve in a KI solution because of the following equilibrium-

    I2(aq) +I-(aq) ------> I3- (aq) where 2 and 3 are subscripts and - is negative charge

    A 0.1M KI solution actually dissolves 12.5g of iodine/L, mmost of which is converted to I3-. Assuming that the concentration of I2 in all saturated solutions is same, calculate the equilibrium constant for the above reaction.

    2. Relevant equations



    3. The attempt at a solution

    0.33g/L is 0.0013 mole and 12.5g/L is 0.0492 mole
    I dont know how to proceed
     
    Last edited: Sep 17, 2010
  2. jcsd
  3. Sep 17, 2010 #2

    Borek

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    Staff: Mentor

    Start writing expression for equilibrium constant. Then think which concentrations you are given, and which you can calculate from the simple stoichiometry.
     
    Last edited by a moderator: Aug 13, 2013
  4. Sep 17, 2010 #3
    K=[I3-]/[I2][I-]

    Actually, I did not understand the problem. A saturated solution of Iodine in water is given to us.
    What is the meaning of 'A 0.1M KI solution actually dissolves 12.5g of iodine/L'
    how does this info help us?
     
  5. Sep 17, 2010 #4

    Borek

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    Staff: Mentor

    Why does the iodides solution contain more iodine than solution in pure water?
     
  6. Sep 17, 2010 #5
    It is so because KI is added to it.
    0.1M KI dissolves 0.0492 mole I2
    but we have only 0.0013 mole I2 so all of it dissolves and converts into I3-
    moles of KI left = 0.1-0.0013
    Is this approach correct?
     
  7. Sep 17, 2010 #6

    Borek

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    Staff: Mentor

    No, there was excess iodine present. It was not dissolved, but it was added as a solid, thus there is much more I3- present.

    How much? That's where the mass of dissolved iodine comes handy.
     
  8. Sep 18, 2010 #7
    excess iodine present in KI solution or saturated solution of Iodine in water?
     
  9. Sep 18, 2010 #8

    Borek

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    No idea what you are asking about. SOLID iodine. Solutions are in contact with the solid. Concentrations given are for the SOLUTIONS.
     
  10. Sep 18, 2010 #9
    I would be thankful to you if you write the concentrations at equilibrium as this question is very vague to me.
     
  11. Sep 18, 2010 #10

    Borek

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    Staff: Mentor

    Concentration of free iodine is identical with the concentration in pure water. Total concentration of iodine dissolved is sum of free iodine and I3-. Simple subtraction will give you concentration of I3-, then simple stoichiometry will tell you how much I- was left.
     
    Last edited by a moderator: Aug 13, 2013
  12. Sep 18, 2010 #11
    Thanks alot. I understood it clearly now.
     
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