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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:
$d=\frac{z}{N_A}\frac{M}{a^3}$
z = no. of formula units per cell
for CsCl type structure z = 2
M = molar mass of ionic solid
N_A = Avogadro's number
a = edge length in cm

Regards