[LawlKake]

I got a couple of questions wrong on an online quiz lately and I'm not sure what I did wrong.

1. Calculate the density of LiF (3 sig dig.) given that the lattice energy is 1034 kJ/mol and assuming rock-salt structure (fcc lattice).
Hint: Use Born-Lande equation to estimate r0 and use that value to calculate density.

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r0 = [ AZ+Z- e^2 (1-1/n) ] / (4pi epsilon not x 1034000) where Z+ Z- =  +1 -1, A = 1.74756, e = 1.602e-19, n=6, dielectric constant in vacuum = 8.854e-12
   = 195.635 pm
edge length  = 2 x root 2 x 195.635
volume = edge length^3
mass = (4 x 6.941 + 4 x 18.998 g/mol) x 1 mol / 6.022x10^23
density = m/v
           = 23.0

I realize that value is much higher than the published density, but I can't see where I went wrong with the math.

http://i.imgur.com/avczs.png (Couldn't post the image as the board seems to automatically resize it smaller)

I've tried 107+32+194+496-326-351 = 152, wrong
Then I realized that I probably had to divide some of the enthalpies by 2 for 1/2Br2 in the formation reaction
So I tried 107+16+97+496-326-351 = 39, wrong

http://i.imgur.com/YxY7i.png

1st attempt: 178+590+1145+244-349-349-796 = 663, wrong
2nd attempt: 178+1145+244-349-349-796 = 73, wrong

EDIT: Upon further thought, would the answers to the last two questions be -741 and -2255 respectively? (By solving for lattice energy U0 = etc, would I use 32 or 16 for enthalpy of Br2(g) formation?)

[LawlKake]

Ah after typing this out and staring at it some I found the problems, I can't seem to delete the thread but a can a mod kindly lock the thread?

Sorry for the trouble!