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Gen Chem Questions

Discussion in 'Pre-Dental' started by Dentalstu02, Jul 27, 2002.

  1. Dentalstu02

    Dentalstu02 Member
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    I have 3 questions about gen chem from the topscore exams. Hopefully they are not hard for those of you who have gone over topscore already. I missed them on the exam however, please help!!

    I don't know what kc is here, does anyone know how to go about this??

    1) When 14.250 moles of PCl5 gas is placed in a 3.00 liter container and comes to equilibrium at a constant temperature, 40.0% of the PCl5 decomposes according to the equation:

    PCl5(g) = PCl3(g) + Cl2(g).

    What is the value of Kc for this reaction?


    2) Given that the standard enthalpy of formation for NO(g) is 90.25 kJ/Mole, calculate the free energy change for the following reaction at 25oC.

    N2(g) + O2(g) = 2 NO(g)

    The absolute entropies of N2 is 192, NO is 211, and O2 is 205...



    3) BaCO3(s) + HCl(aq) (What is the net ionic equation??)


    Any help would be appreciated...thanks!
     
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  3. ohgee

    ohgee Senior Member
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    the answer to your first question is this

    Kc which is the same as Keq (i think)

    will be [Cl2][PCl3]/[PCl5]

    the question says 14.25 mol of PCl5 in 3L solution, so 4.75 M of PCl5 will be reacted.

    It says 40% will decompose to PCl3 and Cl2, so 4.75 M of PCl5 will dissociate to 1.9 M of PCl3 and 1.9M of Cl2 (4.75*40%)

    But 60% remain as PCl5, so 4.75*60%=2.85M of PCl5

    okay, so the Keq will be [1.9][1.9] / [2.85]
     
  4. ohgee

    ohgee Senior Member
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    for # 2

    the free energy equation is
    <G = <H- T<S

    < is change

    okay, well, you got 2M of NO as productso 2*<Hf of NO will be 180.5 (that is your new <H)

    Temperature = 273+25 = 298
    <S = <S product - <S reactant

    therefore, <S = S of 2NO - (S of O2 + S of N2)

    so, 2*211- (192+205) = 25 (your <S)

    put it all together then there is your answer

    for #3,

    i am not sure so i am not going to write anything

    hope this helps, good luck studying
    :)
     
  5. Dentalstu02

    Dentalstu02 Member
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    Thank You soo much Ohgee, the first one is correct and I figured out what I was doing wrong based on what you said....
     
  6. ohgee

    ohgee Senior Member
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    something went wrong, so here it is again...

    for # 2

    the free energy equation is
    <G = <H -T<S

    < is change

    okay, well, you got 2M of NO as product so 2*90.25 = 180.5

    Temperature = 273+25 = 298

    entropy is the entropy of the product minus the reactants, since your have 2 M of NO you need to multiply it by 2...

    therefore, 2*211- (192+205) = 25 (your <S

    put it all together then there is your answer

    oh, since the <H is in KJ, you need to convert T<S to KJ as well..
    <G = 180.5- (298)(25)(1/1000)
     
  7. ohgee

    ohgee Senior Member
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    oh well, it doesn't show,
    just look into your G-chem book for the question, and remember to convert to the correct units!

    good luck
     

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