# $O_2$ 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.

$(a)\;2.68\times10^3\qquad(b)\;2.68\times10^4\qquad(c)\;2.68\times10^6\qquad(d)\;2.68\times10^{10}$

Given
$P = 7.6\times10^{-10}mm$
$\;\;\;=\large\frac{7.6\times10^{-10}}{760}atm$
V = 1 litre
T = 273K
PV = nRT
$\large\frac{7.6\times10^{-10}}{760}\times1 = n\times0.0821\times273$
$n= 4.46\times10^{-14} mole\;of\;O_2$
No. of molecules of $O_2 = 4.46\times10^{-14}\times6.023\times10^{23}$
$\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;=2.68\times10^{10}$