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Homework Help: Need help finding center of mass of a molecule.

  1. Mar 24, 2013 #1
    Figure below shows the shape of a nitric acid (HNO3)molecule and its dimensions. Treating the atoms as particles, findthe center of mass of this molecule.

    a)I know that xcm=m1x1+m2x1+...../Total mass.
    The total mass in this case is 63 but based on the picture I dont know which mass I should plug in for the x values.

    b)ycm=0 because there is no y axis.

    Attached Files:

  2. jcsd
  3. Mar 24, 2013 #2
    The mass for every x value is the mass of the atom at that position.

    What do you mean by "there is no y axis"?
  4. Mar 24, 2013 #3
    so you mean for the 1st one 0.1 nm*1(mass of hydrogen)+0.141nm*16(mass of oxygen)/63?

    theres no y axis means there isnt an angle that goes 90 degrees.
  5. Mar 24, 2013 #4
    What about nitrogen?

    There is a Y axis. And it is even clearly labeled as such.
  6. Mar 24, 2013 #5
    a) 0.1 nm*1(mass of hydrogen)+0.141nm*16(mass of oxygen)+0.282*14(mass of nitrogen)/63
    (should I use the two lengths by nitrogen 0.141nm+0.141nm or just 0.141nm*14g)?

    b) the y axis has a length of zero so when multiplied with the mass of oxygen it becomes zero.
  7. Mar 24, 2013 #6
    What are the distances measured from?

    An axis cannot have a length of zero. An axis is infinite by definition.
  8. Mar 24, 2013 #7
    the distances are measures from the 130 degree angle.
  9. Mar 24, 2013 #8
    And I would think the distances are measured from the origin of the coordinate system, which is at the intersection of the X and Y axes. I advise that before going further, you should find out the coordinates of all the atoms in the molecule.
  10. Mar 24, 2013 #9
    if the origin is O where the y axis is labeled then coordinates are ,H(0,-0.1),N(0,0.141)
  11. Mar 24, 2013 #10
    The molecule has 5 atoms.
  12. Mar 24, 2013 #11
    H(-0.1,0),o(0,0),n(0.141,0), o(0.2,0.12),o(0.2,-0.12)
  13. Mar 24, 2013 #12
    I assume the first number in all the pair is the Y coordinate, correct? It is unusual, but permissible. The first three pairs are OK. The last two, however, are completely wrong.
  14. Mar 24, 2013 #13
    fot the last 2 I did .141cos65 ans added to .141
  15. Mar 24, 2013 #14
    I am sorry, I have misread your message. The coordinates are correct. Now you have all the X's and all the Y's to compute the c. m.
  16. Mar 26, 2013 #15
    thanks alot! , I plugged em into the equation and ended up with 0.131 for x and 0 for y.
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