The density of potassium bromide crystal is `2.75 g cm^(-3)` and the length of an edge of a unit cell is 654 pm. The unit cell of KBr is one of three types of cubic unit cells. How many formula units of KBr are there in a unit cell ? Does the unit cell have a NaCl or CsCl structure ?

Explanation

We know that,
`rho=(N)/(a^(3))[(M)/(N_(A))]`
`N=(rhoxxa^(3)xxN_(A))/(M)`
`=(2.75xx(654xx10^(-10))^(3)xx6.023xx10^(23))/(119)=3.89 ~~4`
Number of mass points per unit cell =4
It is NaCl type crystal, i.e., fcc structure.