Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Ideal GAS LAW

  1. Jan 10, 2012 #1
    1. The problem statement, all variables and given/known data

    Blimps are being considered for use as freight carriers. A scale model rises when filled with helium to a volume of 55.0 dm3. When 1.10 mol He is added to the blimp, the volume is 26.2 dm3. How many more grams of He must be added to make it rise? Assume constant T and P. (4.003 g He = 1 mol He)

    2. Relevant equations

    PV = nRT
    R = 0.8206 ( litres-atm/mole-K )
    1 dm^3 = 1L

    3. The attempt at a solution

    Required volume to fill is 28.8 dm^3 or L

    PV = nRT

    Assume T and P is constant?
    How do we do that? That is my question.
  2. jcsd
  3. Jan 10, 2012 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    You are trying to find n. You want to express n as a function of V. If P and T are constant, what is the relationship? How would you find P/RT (which is constant) from the information provided?

  4. Jan 10, 2012 #3
    They are inversely proportional to each other ( P and T ) ,

    Universal Gas Constant = 8.206 ( Litres - atm / mole - K )

    n = PV/RT

    1 atm is the constant Pressure at sea level but I dont know the temperature?
  5. Jan 10, 2012 #4
  6. Jan 10, 2012 #5


    User Avatar

    Staff: Mentor

    You don't need to know the pressure or temperature; It's enough to know that they are constants.

    In your expression for n above, group all the constants together and replace them by a single new constant. Let's call it k. Now, in the problem statement you're given a particular case for n and V. Find k from that particular case. You can then use this k value to proceed.
  7. Jan 10, 2012 #6
    P/T = nR/V

    Let P/T = k

    k = nR/V

    Given the case that there are 1.10 moles in 26.2 dm^3/L of He

    k = [1.10moles ( 0.8206 ) L-Atm/moles-K] / 26.2L of He

    k = 0.03445267176 Atm/Kelvin

    Constant value gained!

    n = V/R * (k)

    n = 28.8 L / 0.8206 Litres-atm/mole-kelvin * ( 0.03445267176 Atm/Kelvin )

    n = 1.21 moles

    1 mole = 4.003 g HE

    He = 4.84 Grams.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook