The density of CaO is `3.35xx10^(3)kg*m^(3)`. This oxide crystallises in one of the cubic system with a = 480pm. Calculate the number of formula units in the unit cell of the oxide. Which type of cubic system is it?

Explanation

From question,
Density `(rho) = 3.35 gm//cm^(3)`
`a = 4.80 Å`
`M_(CaO)=(40+16)=56`
`rho = (n xx M_(m))/(a^(3))xx N_(A)`
When `n =` No. of molecules per unit cell.
`therefore n =(3.35xx(4.8xx10^(-8))^(3)xx6.023xx10^(23))/(56)=3.98`
or `n ~~ 4`
`4` - units of `CaO` are present in `1` unit cell
No. of `Ca^(2+)` ions `= 4`
No. of `O^(2-)` ions `= 4`
Therefore, cubic system is FCC type.