Use the basic hydrostatic pressure equation to show that the mass of an air column of unit surface area $(1$ m$2)$ and very large height $($say h $=100$ km$)$ is p$0/$g where V is the atmospheric pressure at sea level and g is the acceleration due to gravity $($g $=9\mathrm{.}8$ ms$2).$ Thus, estimate $($a$)$ the total mass of the Earth's atmosphere, assuming the global mean pressure at the surface of the Earth is PS $=984$ hPa due to the variations in elevation of the land $($radius of earth R $=6400$ km$),$ and $($b$)$ the total number of moles of air in the atmosphere.