$1\mathrm{.}2$ Given the following composition of dry air at sea level

Assume that air is an ideal gas and that in the system of interest the

atmospheric pressure is $1\mathrm{.}0$atm and the temperature is $20C.$ The ideal gas

law is given by: $PV=nRT,$ where $R=8\mathrm{.}21{10}^{-5}$atm$-\frac{m^3}{m}ol-K.$

$($a$)$ Compute the density of the dry air at these conditions. Express your

answer in $\frac{g}{m}3.$

$($b$)$ Determine the concentration of argon in air, expressed on a mass

basis, in ppm$.$