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Differential Equation - Brine Solution Entering Tank

  1. Feb 24, 2008 #1
    1. The problem statement, all variables and given/known data

    A tank contains 80 gallons of pure water. A brine solution with 2 lb/gal of salt enters at 2 gal/min, and the well-stirred mixture leaves at the same rate. Find (a) the amount of salt in the tank at any time and (b) the time at which the brine leaving will contain 1 lb/gal of salt.

    2. Relevant equations

    I'm just wondering about (b) really. I know we set S=80 below to solve it, but why?

    3. The attempt at a solution

    The differential equation that gives (a) is

    S=160 - 160*e^(-t/40)

    where S is the amount salt in the tank at any time t.
     
    Last edited: Feb 24, 2008
  2. jcsd
  3. Feb 24, 2008 #2
    If you correctly modeled a diff. eq for this problem and, also correctly solved it to come up with the sol

    S=160 - 160*e^(-t/40), then part b)is not a problem at all. what it is asking u is that when will S(t)=1, and not 80 as you are saying!
    remember S(t) is the amount of salt that the tank contains at any time.
    The diff eq for this problem is

    dS/dt=Ri*Ci- (S*Ro)/(Vo+(Ri-Ro)t) , where

    S--- is the amount of salt in the tank,
    Ri rate in
    Ro rate out
    Ci concentration in
    Vo the initial volume

    EDIT: You haven't actually showed us what u have done at all, remember one of the forums main policy is that you must first show your work, for after the people here to give you hints!!
     
    Last edited: Feb 24, 2008
  4. Feb 24, 2008 #3
    Oh, I'm sorry about that. I'll be sure to put up my work soon. Are you sure that what it's asking though? My notes say that I should get somewhere around 28 minutes.
     
  5. Feb 24, 2008 #4
    A tank contains 80 gallons of pure water. A brine solution with 2 lb/gal of salt enters at 2 gal/min, and the well-stirred mixture leaves at the same rate. Find (a) the amount of salt in the tank at any time and (b) the time at which the brine leaving will contain 1 lb/gal of salt.

    here it is :

    dS/dt=2*2- (S*2)/(80+(2-2)*t)
    dS/dt=4-2S/80, just solve this diff eq, if you haven't gone like this.
     
  6. Feb 24, 2008 #5
    and for the part b) it is just asking you at what time t=? will S(t)=1, like i said.
    NOTE: Next time show your work if you want to recieve any help!!!!
     
    Last edited: Feb 24, 2008
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