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Chem Equilibrium Question

  1. May 3, 2017 #1
    1. The problem statement, all variables and given/known data
    Excess BaCO3 is placed in a container containing 5L of pure water. Given the following info:
    BaCO3 (s) ↔ Ba2+ + CO3 2- and Kc = 5.7E-6
    how many moles of BaCO3 dissolve?

    2. Relevant equations
    Kc=(Ba2+ )(CO3 2- ) = 5.7E-6

    3. The attempt at a solution
    solved for x to get .002387 moles/5L which is incorrect. Dividing by 5L gives me .000477 moles. The correct answer is .012 and I am unsure what I have done wrong. It seemed simple enough. Could someone tell me what to do?
     
    Last edited: May 3, 2017
  2. jcsd
  3. May 3, 2017 #2

    Borek

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    Staff: Mentor

    0.002387 is not number of moles.
     
  4. May 3, 2017 #3
    I forgot to add my last step but I know. It's the moles per liter. If I divide by 5 it gives me .000477 moles which is also not correct.
     
  5. May 3, 2017 #4

    Borek

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    Staff: Mentor

    No, it is not moles. Check your units.

    Randomly juggling numbers is NEVER a good approach to solving problems.
     
  6. May 3, 2017 #5
    Then what is x supposed to be? I was under the impression that the number by which it BaCO3 decreases is equal to the x that each of the products increases by.
     
  7. May 3, 2017 #6

    Borek

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    Staff: Mentor

    So you are solving for x not knowing what it is? That's randomly juggling numbers, I told you it doesn't work.

    You have calculated something in your first post. What it was? Can you post your whole solution?
     
  8. May 3, 2017 #7
    Yeah, all I did was calculate 5.7E-6 = x2 and solved for x which got me to .002387. Isn't its units M/5L because the container contains 5L of water?
     
  9. May 3, 2017 #8

    Borek

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    Staff: Mentor

    No, that's not how units work. Units don't contain numbers (or, if anything, we assume all numbers to be equal to 1). Besides, typically by M we denote molar concentration (which has its own units, mol/L ← an important hint here!), not number of moles.

    What is x intended to mean in this equation?
     
  10. May 3, 2017 #9
    The Kc equation contains (Ba2+ )(CO32- ) and they both gain x per the ice chart so therefore Kc = x2
     
  11. May 3, 2017 #10

    Borek

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    Staff: Mentor

    And what you mean by (Ba2+) (or (CO32-))?
     
  12. May 3, 2017 #11
    Those are the products in the equilibrium. But I figured it out, anyways. I was dividing by 5L of water when I should have been multiplying by 5L because I needed to eliminate that 5L in the denominator and get moles by itself!
     
  13. May 3, 2017 #12

    Borek

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    Staff: Mentor

    These are CONCENTRATIONS. And concentration has units of mol/L. Per ONE liter, not per 5 liters.

    When you divided by 5 L you got the result in mol/L2 units - these don't make sense, but you have never checked so you were not able to spot the mistake.
     
  14. May 3, 2017 #13
    I know, thank you for the hint.
     
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