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: Rotational KE Problem

  1. Jan 14, 2006 #1
    In a crude model of a rotating diatomic molecule of chlorine (Cl2), the two Cl atoms are 2.00x10^-10 m apart and rotate about their center of mass with angular speed = 1.40x10^12 rad/s. What is the rotational kinetic energy of one molecule of Cl2, which has a molar mass of 70.0 g/mol?

    I have tried this problem but I really don't even know where to start..
     
  2. jcsd
  3. Jan 14, 2006 #2

    siddharth

    User Avatar
    Homework Helper
    Gold Member

    What is the forumla for Rotational Kinetic energy of a an object?
     
  4. Jan 14, 2006 #3
    KE=1/2* I* (angular speed)^2

    I=1/12*m*L^2

    would mass=140 g?
     
  5. Jan 14, 2006 #4

    siddharth

    User Avatar
    Homework Helper
    Gold Member

    Why have you chosen I = 1/12* m*l^2? Are you approximating this as a rod of length L? I don't think that's a good idea becuase most of the mass is concentrated in the atom, not the bond. So I think taking this as two sepearate Cl atoms rotating about the COM is a better idea

    Also, the mass will not be 140g. The mass of 1 mol of Cl2 is 70 grams. And you know 1 mol of Cl2 contains around 6.023*10^23 molecules. So what is the mass of 1 molecule?
     
  6. Jan 14, 2006 #5
    ok i got the mass of one to be 1.16E-22

    But then what equation for moment of inertia would I use?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook