The well known mineral fluorite is chemically clacium fluoride. Its is known that in one unit cell of this mineral there are 4 `Ca^(2+)` ions and `8 F^-` ions and that of `Ca^(2+)` ions are arranged in a fcc lattice. The `F^-` ions fill all the tetrahedral holes in the face centred cubic lattice of `Ca^(2+)` ions. the edge of hte unit cell is `5.46xx10^-8 cm` in length. the density of the solid is `3.18 gcm^-3`. use this information to calculate the Avagadro's number. (Molar mass of `CaF_2=70.08 g mol^-1`).