Dear student,

To find the density molar mass of ionic solid AB should be known. You can use the formula given below to find the density:
$d=\frac{z}{{N}_{A}}\frac{M}{{a}^{3}}\phantom{\rule{0ex}{0ex}}$
z = no. of formula units per cell
for CsCl type structure z = 2
M = molar mass of ionic solid