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

`rho =(Z xx M)/(N_(o)xx a^(3))`
`therefore z = (d xx a^(3)xx N_(o))/(M)`
`= (2.75xx(654xx10^(-10))^(3)xx6.023xx10^(23))/(119)`
`= 3.89 ~~4`
Number of formula units = 4
It is NaCl type crystal (FCC).