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Please help Gen Chem TopScore problem!

Discussion in 'DAT Discussions' started by Dentista08, Dec 2, 2008.

  1. Dentista08

    Dentista08 looking for a practice!
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    I just dont get where did 2.854 came from? How do we know the concentrations of the products? Please see attachment
    The answer is A not b
     

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  3. chessxwizard

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    So in this question, they give you the moles of starting reactant, volume of the container, and the percent of products that are formed. First of all, you need to find the molarity of the reactants:

    14.250 moles PCl5 / 3 L container = 4.75M PCl5
    And you know that 40% of the reactants are formed into products:

    .40 x 4.75 M = 1.9 M of each PCl3 and Cl2 are formed in the reaction.

    Then the reaction proceeds in the way written... create an ICE table and plug in the values, keeping everything written in moles/L:

    PCl5 -> PCl3 + Cl2
    initial 4.75 0 0
    change -x x x
    equilib. 4.75-x x x

    But we know that x = 1.9M, since that's how much is formed at the end of the reaction! So for the Kc value, we just plug in x into the equilibrium values:

    PCl5 @ Equilibrium: 4.75-1.9 = 2.85M
    PCl3 @ Equilibrium: 1.9M
    Cl2 @ Equilibrium: 1.9M

    Therefore, Kc = [Cl2][PCl3]/[PCl5]
    = [1.9^2]/[2.85], choice A.
     
  4. Dentista08

    Dentista08 looking for a practice!
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    :thumbup: totally get it now!!! thank you
     
  5. sciencegod

    sciencegod Super Member
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    i dont get it. if 40% gets det decomposed. doesnt that mean that 40% which is 1.9 M should be split into the two products and each one should only be .95?
     
  6. Svart Aske

    Svart Aske is stabbing his eyes out
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    Each mole of PCl5 splits into a mole of PCl3 and Cl2 each. This is basic stoichiometry; think back to the Initial Change Equilibrium (ICE) chart in gen chem.
     

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