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: Density of an Ionic Crystal

  1. Nov 29, 2008 #1
    1. The problem statement, all variables and given/known data
    The distance between the Li+ and Cl- ions in a LiCl is .257 nm. Use this and the molecular mass of LiCl 42.4 g/mol to compute teh density of LiCl.

    2. Relevant equations

    3. The attempt at a solution
    I am surprisingly having a lot of difficultly with this problem.
    The LiCl crystal is face centered (as far as I can tell) so there is an entire LiCl and either a Li or Cl in the center (I think Cl, but I'm not sure).

    If I follow the example in the book I can simply find the answer, but it doesn't make sense.

    [tex]\rho[/tex] = m/V = 42.4 / (2 * 6.02 x 10^23 * r0^3)

    This works but I don't understand it. Here is how I attempted to do it...

    [tex]\rho[/tex] = m/V = (42.4 + 1/2 42.4) / (6.02 x 10^23 * r0^3)

    I am assuming that since the problem doesn't give the information I can't look up whether or not Cl is in the middle or even the mass of Li and Cl separately. So I took 1/2 the mass of LiCl hoping that would average out. Unfortunately that gives an incorrect answer.

    Does anyone have a way to explain why 1/2 is the correct factor in this case? It seems strange to me.
  2. jcsd
  3. Nov 29, 2008 #2
    sorry, i seem to have confused fcc with bcc...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook