The compound MO (molar mass= 100 g mol ^-1) has zinc-blende closest unit cell. If the radius of M²⁺ is 69.4 pm, the density of MO will be

r_a = r_c /0.225 = 69 .4 pm/0.225 =308.4 pm;

$a=2\sqrt{2} r_a=2\sqrt{2} \left(308.4 pm\right) =872.4 pm$

$\rho =\frac{NM}{N_{\mathrm{A}}{a}^3}=\frac{4\left(100{\mathrm{gmol}}^{-1}\right)}{\left(6.022\times {10}^{23}{\mathrm{mol}}^{-1}\right){\left(872.4\times {10}^{-10}\mathrm{cm}\right)}^3}=\frac{400}{399.84}\mathrm{g}{\mathrm{cm}}^{-3}=1.0\mathrm{g}{\mathrm{cm}}^{-3}$