The number of molecules present in 1 mL of gas or vapour at STP is:

 $$\begin{gathered} \mathrm{Loschmidt}\text{'}\mathrm{s} \mathrm{number}\left({\mathrm{n}}_{\mathrm{o}}\right) \mathrm{is}: \mathrm{No}.\mathrm{of} \mathrm{molecules} \mathrm{present} \mathrm{in} \mathrm{one} \mathrm{c}.\mathrm{c} \mathrm{or} \mathrm{mL} \mathrm{at} \mathrm{STP} \mathrm{conditions}. \\ {\mathrm{n}}_{\mathrm{o}}=2.687\times {10}^{19}\mathrm{molecules} \mathrm{per} \mathrm{mL} \end{gathered}$$