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Homework Help: Mass Balance Problem - Help Please!

  1. Feb 27, 2008 #1
    1. The problem statement, all variables and given/known data

    A tank used to prepare 10% salt solution was shut down for the day. The next day, you were told that the effluent concentration should be 5 wt %. The tank volume is 1000 m^3, and it is completely filled with 10% solution. You decide to run pure water and remove the contents at the same rate until the concentration drops from 10% to 5% and readjust the new steady state flow rates of salt and water. How long it will take to decrease the concentration in the tank if you feed water at 100 lt/hour?

    2. Relevant equations

    Mass Balance Equation: dm(total)/dt=dm(1)/dt + dm(2)/dt + ...

    3. The attempt at a solution

    Firstly, it is not a steady state respect to salt, salt concentration is changing with time.

    I tried to write mass balance (difference) equation respect to salt:

    dm(salt)/dt= 0 - ?

    Now, 0 (zero) because salt is not being added to system, conversely, it's concentration is lowerin'.

    Please guys, help me. It's very important.

    Thanks from now!
  2. jcsd
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