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Homework Help: Using the Ideal Gas Law

  1. Dec 14, 2016 #1
    1. The problem statement, all variables and given/known data

    I am not asking for the answer to an example, rather how the book got some numbers. The problem is an example from the book and shows me the solution but does not show the steps.

    Given: The compressed air tank has a volume of .84 ft^3. The temperature is 70 F and the atmospheric pressure is 14.7 psi (abs).

    Find: When the tank is filled with air at a gage pressure of 50 psi, determine the density of the air and the weight of air in the tank.)

    2. Relevant equations
    I am told to use the ideal gas law, ρ = p/RT.

    3. The attempt at a solution

    Then the book says,

    ρ = (50 lb/in2 + 14.7 lb/in2)(144 in2/ft2) / (1716 ft * lb/slug * °R) [(70 + 460) ° R]

    So I understand where the (50 lb/in2 + 14.7 lb/in2) is coming from, they are just adding the gage pressure as well as the atmospheric pressure.

    BUT, I don't understand where the (144 in2/ft2) came from, as well as the (1716 ft * lb/slug * °R), and [(70+460) °R].

    Any help will be appreciated.
  2. jcsd
  3. Dec 14, 2016 #2


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    Staff: Mentor

    The units of that 144 term are a hint.

    What value would you have used for R in the formula ##\rho=p/RT## ? You would have probably used a google search to determine R. What name is R known as?

    How would you convert 70° F to degrees Rankine to use in your Gas Law formula?
  4. Dec 14, 2016 #3
    I was going to use 8.314 K-1mol-1 since that is the gas constant for air.

    I looked through my textbook and found that °R = °F + 459.67, so I would do 70°F + 460 = °R.

    Would I do something with the volume to get the 144? I am not even sure what that number is doing there since the equation just says absolute pressure should be the numerator.
  5. Dec 14, 2016 #4


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    Staff: Mentor

    What is the result if you take the square root of everything in the brackets here: ##\left( 144\ \dfrac {in^2}{ft^2} \right)##
  6. Dec 14, 2016 #5
    You would get 12 in/ft.
  7. Dec 14, 2016 #6


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    Staff: Mentor


    Choose whichever of the alternative forms best suits you and your data, e.g.,
    from http://www.engineeringtoolbox.com/individual-universal-gas-constant-d_588.html
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