What is the number of unit cells present in $3.9 \ g$ of potassium if it crystallizes in $BCC$ structure?

The density of $KBr$ is $2.75 \, g \, cm^{-3}$ and the unit cell edge length is $654 \, pm$. Given the atomic masses $K = 39$ and $Br = 80$, what is the crystal structure of $KBr$?

The atomic radius of strontium $(Sr)$ is $215 \, pm$ and it crystallises in a cubic closed packed $(ccp)$ structure. The edge length of the cube is .............. $pm$.

Metal $M$ crystallizes into a $FCC$ lattice with the edge length of $4.0 \times 10^{-8} \ cm$. The atomic mass of the metal is $........ \ g/mol$. (Nearest integer). (Use: $N_{A} = 6.02 \times 10^{23} \ mol^{-1}$,density of metal,$d = 9.03 \ g \ cm^{-3}$)

The unit cell of copper corresponds to a face-centered cubic $(FCC)$ lattice with an edge length of $3.596 \, \mathring{A}$. The calculated density of copper in $kg / m^{3}$ is ....... .
[Molar mass of $Cu = 63.54 \, g/mol$; Avogadro Number $= 6.022 \times 10^{23} \, mol^{-1}$]

$A$ crystalline solid of a pure substance has a face-centred cubic structure with a cell edge of $400 \ pm$. If the density of the substance in the crystal is $8 \ g \ cm^{-3}$, then the number of atoms present in $256 \ g$ of the crystal is $N \times 10^{24}$. The value of $N$ is