An ionic compound MX has rock-salt structure. If r_c= 120 pm and r_a= 180 pm, then the number of molecules of MX in a cube of edge length of 1 cm is about

The edge length of unit cell is $a=2\left(r_{\mathrm{a}}+r_{\mathrm{c}}\right)=2(180+120)\mathrm{pm}=600\mathrm{pm}=600\times {10}^{-10}\mathrm{cm}$

The volume of unit cell is $v={\left(600\times {10}^{-1}\mathrm{cm}\right)}^3=2.16\times {10}^{-2}\mathrm{cm}$ 

The number of molecules in $V=1{\mathrm{cm}}^3\text{ is }N=\left(\frac{4\text{ molecules }}{2.16\times {10}^{-22}{\mathrm{cm}}^3}\right)(1\mathrm{cm}{)}^3=1.85\times {10}^{22}\text{ molecules }$