Question

CsCl has a cubic structure.Its density is 3.99kg/m^3.What is the distance between Cs+ and Cl- ions?(atm mass of cs is 133)

Answer 1

Density=M.mass*Z/a3*Na  (Z=2 For CsCl..bcc)

  3.99=168.5*2/a3*6.022*1023

or 

168.5*2/6.022*1023*3.99= a3

Answer 2

density=Z*M/N*a^3
(z=1 since primitive cubic lattice
M= atomic mass =133
a=edge length
N=avogadro no.)
a^3=Z*M/N*density
      =1*133/6.022*10^23*3.99
      =5.53*10^-23