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Homework Help: Tank concentration-of-chlorine problem (Dif. Eq.)

  1. Jul 29, 2010 #1
    1. The problem statement, all variables and given/known data

    A tank with a capacity of 400 L is full of a mixture of water and chlorine with a concentration of 0.05 g of chlorine/liter. The chlorine concentration is to be reduced by pumping in fresh water at the rate of 4 liters/second. The mixture is kept stirred and pumped out a rate of 10 liters/second. What is the concentration of the chlorine in the tank 15 seconds later?

    3. The attempt at a solution

    Set x = the amount of chlorine in the main tank.

    Well there isn't any chlorine entering the main tank, so the mixture going in would be 4, and the amount going out would be 4*(x / 400). So my setup is:

    (dy/dt) = 4 - 4*(x / 400)

    And I don't really know where to go from there. I don't even know if my setup is correct. I'm still trying to get the intuition of differential equations, lol.

  2. jcsd
  3. Jul 29, 2010 #2


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    That isn't a very good start. You haven't even said what 'y' is supposed to be. Here's a hint. Let V be the volume of solution contained in the tank. Can you express V as a function of t? Now can you express dx/dt in terms of V and x?
  4. Jul 29, 2010 #3
    I'm sorry, I meant:

    dx/dt = 4 - 4*(x / 400)

    Sorry for my mistake. For my volume function:

    V(t) = 400x + 4 - 10t(x / 400)
    dV/dt = 4 - x / 100

    Am I on the right track?

    EDIT: And x = 5 / 100 when t = 0, correct?
  5. Jul 29, 2010 #4


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    Mmm. Not really. I think you can express V(t) without any reference to x. Can't you? Just concentrate on that for now.
  6. Jul 29, 2010 #5
    V(t) = 0.05 - (0.05 / 400)*6*t ?

    I'm sorry, I'm still trying to get the intuition of differential equations. I know the result will pretty much always be a function, but it's difficult for me to comprehend what a differential is equal to. I know an equation contains two of the same quantities, but an equation of differentials is outside my usual scope of comprehension. So I don't really know how to set it up. My professor is at a science convention in D.C. for a few days and I'm really trying to learn these without his guidance, lol. I hope this forum's denizens are the patient types :)

    EDIT: 6 not 10 because of the 4 liters going in, right?
    Last edited: Jul 29, 2010
  7. Jul 29, 2010 #6


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    I'm patient. But I don't stay up all night either. Look V(0)=400 L, right? And then, yes, V(t) goes down by 6 L per second. I think you are overcomplicating this. Tell me quick. What's V(t)?
  8. Jul 29, 2010 #7
    I'm sorry, nevermind I never figured it out. Thank you for your help.
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