Answer
$Density=2\frac{g}{cm^3}$
Work Step by Step
We know that edge length a is
$a=2\times 314=628pm$
As Molar mass of $KCl=74.5\frac{g}{mol}$
mass of 4 molecules $=\frac{4\times 74.5}{6.023\times 10^{23}}g$
Now $Density=\frac{4\times 74.5}{6.023\times 10^{23}\times (628\times10^{-10} )^3}$
$Density=2\frac{g}{cm^3}$
