AB has rock-salt (A : B :: 1 : 1) structure, i.e., fcc structure (n = 4) and formula weight of AB is 6.023 y g having closest distance A – B ${\mathrm{y}}^{1/3}$ nm. Therefore, edge length of unit cell
$\left(\mathrm{a}\right)=2({\mathrm{A}}^{+}+{\mathrm{B}}^{-})=2\times {\mathrm{y}}^{1/3}\times 10^{-9}\mathrm{m}$
So density of AB $=\frac{\mathrm{n}\times \mathrm{m}\mathrm{o}\mathrm{l}.\mathrm{w}\mathrm{t}.}{\mathrm{A}\mathrm{v}.\mathrm{n}\mathrm{o}.\times \mathrm{V}}=\frac{\mathrm{n}\times \mathrm{m}\mathrm{o}\mathrm{l}.\mathrm{w}\mathrm{t}.}{6.023\times 10^{23}\times {\mathrm{a}}^3}$
$=\frac{4\times 6.023\times \mathrm{y}\times 10^{-3}}{6.023\times 10^{23}\times (2{\mathrm{y}}^{1/3}\times 10^{-9}{)}^3}$ (mol. wt. in kg)
$=\mathrm{5.0 }\mathrm{k}\mathrm{g}{\mathrm{m}}^{-3}$