The density of KBr is ` 2.75 " g cm^(-3)` , The length of edge of the unit cell is 654 pm. Predict, the type of cubic lattice to which unit cell of KBr belongs ` ( N_(0) = 6.023 xx 10^(23) " mol" ^(-1) ` , At mass : K = 29 , Br = 80)

Explanation

For cubic crystals, `rho=(ZxxM)/(a^3xxN_0)`
or `therefore Z=(rhoxxa^3xxN_0)/M=((2.75"g cm"^(-3))(654xx10^(-10) cm)^3 xx(6.023xx10^23 "mol"^(-1)))/((39+80)"g mol"^(-1)) =3.89=4`
Thus, there are four formula units of KBr present per unit cell. Hence , it has face-centred cubic lattice (similar to that of NaCl).