Density of an Ionic Crystal

  • Thread starter roeb
  • Start date
  • #1
107
1

Homework Statement


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.


Homework Equations





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.
 

Answers and Replies

  • #2
107
1
sorry, i seem to have confused fcc with bcc...
 

Related Threads on Density of an Ionic Crystal

  • Last Post
Replies
8
Views
30K
R
  • Last Post
Replies
3
Views
6K
  • Last Post
Replies
13
Views
13K
  • Last Post
Replies
0
Views
2K
  • Last Post
Replies
7
Views
3K
  • Last Post
Replies
15
Views
7K
  • Last Post
Replies
2
Views
2K
  • Last Post
Replies
0
Views
2K
  • Last Post
Replies
9
Views
3K
Top