Number of molecules in a volume of $4 \mathrm{~cm}^3$ of a perfect monoatomic gas at some temperature T and at a pressure of 2 cm of mercury is close to ? (Given, mean kinetic energy of a molecule (at T ) is $4 \times 10^{-14} \mathrm{erg}, \mathrm{g}=980 \mathrm{~cm} / \mathrm{s}^2$, density of mercury $= \left.13.6 \mathrm{~g} / \mathrm{cm}^3\right)$