With a vacuum pump we can go to a very low pressure of $1.0\cdot 10^{-3}$ torr. Calculate then the number of molecules in $1.00$ mL of this gas at $20^oC$ !
760 torr = 1 atm number of molecules = $n\cdot6\cdot 10^{23}$ (n: number of moles)
$P$ $=$ $\frac{10^{-3}}{760}atm$ $1 mL$ $=$ $10^{-3}$ L $n$ $=$ $\frac{PV}{RT}$ $=$ $\frac{10^{-3}10^{-3}}{760\cdot 0.082\cdot 293}$ Number of molecules = $n\cdot 6\cdot 10^{23}$ $=$ $\frac{10^{-3}10^{-3}6\cdot 10^{23}}{760\cdot 0.082\cdot 293}$ $=$ $3.29\cdot 10^{13} $ molecules
