Given the following composition of dry air at sea level

$\backslash$table$[[$Component$,$Mole Fraction $(\%)],[O_2,20\mathrm{.}95],[N_2,78\mathrm{.}09],[Ar,0\mathrm{.}93],[C{O}_2,0\mathrm{.}04]]$

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 $m\frac{g}{L}.$

b$)$ Determine the concentration of carbon dioxide in air, expressed on a mass basis, in ppm$.$