Question

A good vaccum produced in common laboratory apparatus corresponds to ${10}^{-6}mm$ pressure at ${25}^{o}C$. Calculate number of molecules per $cc$ at this pressure and temperature.

Answer 1

$P={10}^{-6}mm$ of $Hg$

$T={25}^{o}C=298K$

$PV=nRT$

$P=\frac{n}{V}RT$

$\frac{n}{V}=\frac{P}{RT}=\frac{\frac{{10}^{-6}}{760}}{0.0821\times 298}=5.378\times {10}^{-11}$

$\therefore$ no. of molecules per cc $=3.23\times {10}^{10}$