Answer
$2.16\times10^3kg/m^3$.
Work Step by Step
The volume occupied by each atom is $(0.282 nm)^3 $. Na and Cl occur in equal amounts, so the density is the average mass of
Na and Cl, divided by this volume.
$$m_{average}=4.855\times10^{-26}kg$$
Now calculate the density.
$$\rho=\frac{4.855\times10^{-26}kg}{(0.282\times10^{-9}m)^3}=2.16\times10^3kg/m^3$$
