$2-$ Calculate the energy for vacancy formation in silver, given that the equilibrium number of vacancies at $800C(1073K)$ is $3\mathrm{.}6{10}^{23}{m}^{-3}.$ The atomic weight and density $($at $800C)$ for silver are, respectively, $107\mathrm{.}9\frac{g}{m}ol$ and $9\mathrm{.}5\frac{g}{c}{m}^3.$