Answer
$P = 3.49 \times 10^3 atm$
Work Step by Step
We use the equation of ideal gas
$P = {NkT } $
Where
$N=8.43\times10^{28} m ^{-3}$
$k = 1.38 \times 10^{-23} J/K$
$T= 300K$
So
$P = (8.43\times10^{28} m ^{-3})(1.38 \times 10^{-23} J/K)(300K) $
$P = 3.49 \times 10^8 Pa$
Change unit to atm assuming $1 Pa = 1 \times 10^{-5} atm$, so
$P = 3.49 \times 10^3 atm$
