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Homework Help: Combining of two gases within an airflow

  1. Jun 14, 2010 #1
    1. The problem statement, all variables and given/known data
    not really a question, just stuck with something.
    An airflow of 300 CFM has a contains 0.0387% CO2 (carbon dioxide-naturally occurring in air) flows through two ends of an rectangular open container from an exhaust fan located at the bottom.

    Within the container 100% CO2 can be added through a controllable valve. This air will mix instantly with the passing .0387 300CFM airflow.

    How much CFM of the 100%CO2 is required to add to the airflow to create a mixture of gases that = 0.5% CO2

    2. Relevant equations

    3. The attempt at a solution
    I tried setting up a simple addition formula:

    [(300CFM)/(.0387% CO2)] + [(x)/(100% CO2)] = (300+XCFM)/?

    I'm not sure if this is even close to being right, even if it is...i don't remember how to find the new percentage of CO2 after adding a certain volume.

    Maybe if the air was constant it would be easier for me...i just can't get my head around the whole "flowing" concept.
  2. jcsd
  3. Jun 14, 2010 #2


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    Let 'x' be the airflow of 100% CO2 added. The airflow of CO2 due to the fan is 300 CFM (ft^3/min) times 0.0387%. The total airflow after the addition of the 100% CO2 will be 300 + x. 5% of that is the concentration of CO2 after the addition. The same amount of air + 100% CO2 enters the chamber as exits. Solve for x.
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