Spray drying and other procedures for manufacturing small particles can produce particles as small as $2$

to $5$ microns. Calculate the external surface area of non$-$porous spherical particles of $2$ microns diameter.

What size particles would be necessary if the external surface is to be $100\frac{m^2}{g}?$ The density of the

particles is $2\mathrm{.}0\frac{g}{c}{m}^3,1$ micron $={10}^{-4}cm={10}^{-6}m.$