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Math Help - Finding how much chemical is left over

  1. #1
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    Finding how much chemical is left over

    A 500 liter tank originally contains 300 liters of liquid in which there's dissolved 50 grams of some chemical. Liquid containing 30 grams per liter of the dissolved chemical flows into the tank at the rate of 4 liters/min. The mixture is uniform and it flows out at a rate of 2 liters/min. How much of the chem is inside at the time it overflows?

    Here's my work:

    There is 300+2t liters of liquid in the tank. I'm going to simplify liters with L.

    Q(t) = 4 \frac{L}{min} x 30 \frac{grams}{L} , Q(0) = 50

    Q(t) = 2 \frac{liters}{min} x \frac{Q(t)}{300+2t}

    Q'(t) = 120 - \frac{2Q(t)}{300+2t}

    = 120 - \frac{2Q(t)}{2(150+t)}

    = 120 - \frac{Q(t)}{150+t}

    Q'(t) + Q(t)\frac{1}{150+t} = 120 Linear Equation.

    <br />
e^{\int\frac{1}{150+t}dt

    = e^{ln|150+t|}

    = 150+t
    <br />
Q'(t)(150+t) + \frac{(150+t)Q(t)}{150+t} = 120(150+t)

    Q'(t)(150+t)+ Q(t) = 18000+120t

    \int [Q(t)(150+t)]'dt = \int 18000 dt + \int 120t dt

    Q(t)(150+t) = 18000t + 60t^2 + C

    Q(t) = \frac{18000t}{150+t}+\frac{60t^2}{150+t}+\frac{C}{  150+t}

    Use initial condition Q(0) = 50.

    50 = \frac{C}{150}

    7500 = C

    Q(t) =  \frac{18000t}{150+t}+\frac{60t^2}{150+t}+\frac{750  0}{150+t}

    Determining the time of overflow:

    300+2t = 500
    2t = 200
    t = 100

    Sub in for t.

    Q(t) =  \frac{18000(100)+60(100)^2+7500}{150+(100)}

    = 9,630 \frac{grams}.
    Last edited by A Beautiful Mind; September 24th 2010 at 11:43 PM. Reason: I forgot to put t^2 instead of t. But it didn't affect my final answer, as I used a calculator with that in mind.
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  2. #2
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    Quote Originally Posted by A Beautiful Mind View Post
    A 500 liter tank originally contains 300 liters of liquid in which there's dissolved 50 grams of some chemical. Liquid containing 30 grams per liter of the dissolved chemical flows into the tank at the rate of 4 liters/min. The mixture is uniform and it flows out at a rate of 2 liters/min. How much of the chem is inside at the time it overflows?

    Here's my work:

    There is 300+2t liters of liquid in the tank. I'm going to simplify liters with L.

    Q(t) = 4 \frac{L}{min} x 30 \frac{grams}{L} , Q(0) = 50

    Q(t) = 2 \frac{liters}{min} x \frac{Q(t)}{300+2t}

    Q'(t) = 120 - \frac{2Q(t)}{300+2t}

    = 120 - \frac{2Q(t)}{2(150+t)}

    = 120 - \frac{Q(t)}{150+t}

    Q'(t) + Q(t)\frac{1}{150+t} = 120 Linear Equation.

    <br />
e^{\int\frac{1}{150+t}dt

    = e^{ln|150+t|}

    = 150+t
    <br />
Q'(t)(150+t) + \frac{(150+t)Q(t)}{150+t} = 120(150+t)

    Q'(t)(150+t)+ Q(t) = 18000+120t

    \int [Q(t)(150+t)]'dt = \int 18000 dt + \int 120t dt

    Q(t)(150+t) = 18000t + 60t^2 + C

    Q(t) = \frac{18000t}{150+t}+\frac{60t^2}{150+t}+\frac{C}{  150+t}

    Use initial condition Q(0) = 50.

    50 = \frac{C}{150}

    7500 = C

    Q(t) = \frac{18000t}{150+t}+\frac{60t^2}{150+t}+\frac{750  0}{150+t}

    Determining the time of overflow:

    300+2t = 500
    2t = 200
    t = 100

    Sub in for t.

    Q(t) = \frac{18000(100)+60(100)^2+7500}{150+(100)}

    = 9,630 \frac{grams}.
    Looks OK.

    You can check answers here: solve dx&#47;dt &#43; x&#47;&#40;150 &#43; t&#41; &#61; 120 and x&#40;0&#41; &#61; 50 - Wolfram|Alpha
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