1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Root Mean Square Speed Units Question

  1. Jan 21, 2014 #1
    1. The problem statement, all variables and given/known data

    "At 273 K and 1.00x10^-2 atm, the density of a gas is 1.24x10^-5 g/cm^3.

    A.) Find the Vrms for the gas molecules
    B.) Find the molar mass and identify the gas (Choose from H2, He, H20, N2, O2, or CO2)"

    2. Relevant equations

    Vrms = √(3RT/Mm)
    pV = nRT

    3. The attempt at a solution

    n = mass/Mm
    pV = mass/Mm *RT
    Mm = mass*RT/(pV)
    Mm = ρRT/p

    Vrms = √(3RT/(ρRT/p)) = √(3p/ρ)

    So I have the solution up until this point, the main thing I am worried about is the units of my answer. I have pressure in atm, which is some form of Force/Area. I have density in g/cm^3. So pressure/density will be some form of (distance/time)^2, which of course is taken care of with the radical. The only thing I can not come up with is what exact units it will be in. Any help here would be greatly appreciated. Thanks in advance.
  2. jcsd
  3. Jan 21, 2014 #2
    A rule of thumb is that if you use SI units for every term in an expression, you can expect the answer to be in SI units. You can do a dimensional analysis to verify this as well. Converting atm to Pascals and g/cm^3 to Kg/m^3, should give you m/s for Vrms.
  4. Jan 21, 2014 #3
    Ok, that makes sense. Thanks a lot!
  5. Jan 22, 2014 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Since ideal gas calculations can sometimes involve a mixture of different units, the following article gives values of R in several different unit combinations:

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

Have something to add?
Draft saved Draft deleted