The compound MX (molar mass= 100 g mol^-1) occurs in zinc blende structure. If r_c= 80 pm and r_a= 120 pm, its density will be

We have $\rho =\frac{4\left(M_{\mathrm{MX}}/N_{\mathrm{A}}\right)}{{\left[4\left(r_{\mathrm{a}}+r_{\mathrm{c}}\right)/\sqrt{3}\right]}^3}=\frac{3\sqrt{3}\left(M_{\mathrm{MX}}/N_{\mathrm{A}}\right)}{16{\left(r_{\mathrm{a}}+r_{\mathrm{c}}\right)}_3}$ $=\frac{\left(3\sqrt{3}\right)\left[\left(120\mathrm{g}{\mathrm{mol}}^{-1}\right)/\left(6.022\times {10}^{23}{\mathrm{mol}}^{-1}\right)\right]}{16{\left[(120+80)\times {10}^{-10}\mathrm{cm}\right]}^3}$ $=\frac{\left(3\sqrt{3}\right)\left(20\times {10}^{-23}\right)\mathrm{g}}{16\times 8.0\times {10}^{-24}{\mathrm{cm}}^3}=8.11\mathrm{g}{\mathrm{cm}}^{-3}$