# $O_2$ is present in one litre flask at a pressure of $7.6\times10^{-10}mm$ of Hg. Calculate the number of $O_2$ molecules at $0^{\large\circ}$C.
$(a)\;2.68\times10^3\qquad(b)\;2.68\times10^4\qquad(c)\;2.68\times10^6\qquad(d)\;2.68\times10^{10}$