KBr has fcc str.The density of KBr is 2.75 g/cm cube.Find the distance b/w K+ and Br- .
For PbS
M= 239g
Z= 4.
Let distance between ions = (R++R-)
d= 2.75g/cm
NA=6.023x1023.
Br- is present at corners while K+ is present at edge centers.
so
(2R++2R-)=a
or
2(R++R-)=a.................(1)
we know that...
Form e.q.(1)
on Substituting the values and calculating you will get the value of (R++R-).
Good luck.