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Homework Help: Decay problem

  1. Feb 10, 2010 #1
    There is a tank that contains 10,000 gallons of a weak salt solution. The salt concentration is 1300 parts per million. Now we start adding 130 gallons per minute of water and also remove 130 gallons per minute of salt solution. Thererore the salt solution concentration gradually decreases to zero parts per million. Assume the tank is agitated and well mixed.

    How long does it take the salt solution concentration to decrease to 100 parts per million?

    How long ....... to 10 parts per million?

    How long ....... to 1 part per million?


    I tried a decay solution as a logarithmic function ( which I believe is not quite correct ) but couldn't come up with correct calculus method.

    Can someone offer a calculus & non-calculus solution ?

    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Feb 10, 2010 #2

    tiny-tim

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    Hi eldrick! :wink:

    Show us what you've tried, and where you're stuck, and then we'll know how to help! :smile:
     
  4. Feb 10, 2010 #3
    I tried this initially as a non-calculus solution ( but told not quite correct ) :

    initial salt content is

    10,000 * 1300/1,000,000 = 13 units

    we want time salt concentration is

    1) 10 per million or total salt content is

    10,000 * 10/1,000,000 = 0.1 units

    2) 1 per million or total salt content is

    10,000 * 1/1,000,000 = 0.01 units

    how do we find the answers ?

    - 0'00 : total salt content is
    10,000 * 1300/1,000,000 = 13 units

    - 1'00 : total salt content is
    (10,000 * 1300/1,000,000) - (130 * 1300/1,000,000) = 1300/1,000,000 * (10,000 - 130) = 12.831 units

    without wasting time on endless factorising :

    at any given time t, total salt is

    13 * ( (10,000 - 130)/10,000 )^t

    or simplify

    13 * 0.987^t

    so, for answers

    1 ) 0.1 = 13 * 0.987^t

    -> t = 371.987 minutes

    1 ) 0.01 = 13 * 0.987^t

    -> t = 547.954 minutes

    The problem appears to be initial formula which is based on salt content at 1 minute & not instantaneous
     
  5. Feb 10, 2010 #4

    tiny-tim

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    Hi eldrick! :smile:

    (try using the X2 tag just above the Reply box :wink:)

    I haven't completely checked your other figures, but yes your 0.987t is correct.

    (the calculus method, for the proportion p, would have been dp/dt = -0.013p)
     
  6. Feb 10, 2010 #5
    Very kind of you Tim

    I'd appreciate it if you would show me how you obtained the differential
     
  7. Feb 11, 2010 #6

    tiny-tim

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    dp/dt = -0.013p ?

    A fixed proportion of the water leaves per second.

    So a fixed proportion of the salt leaves per second.

    That's English for "dp/dt is a constant times p". :smile:
     
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