STP (Standard Temperature and Pressure) has the following conditions:
Temperature = 273.15 K = 0°C
Pressure = 101325 Pa = 101.325 KPa = 1 atm
We also know that 1 mole = 6.022x10^23 molecules
Using the ideal gas equation: PV=nRT
n/V = P/RT
molecules/V = P*6.022x10^23/RT
molecules/V = 101325 Pa (6.022x10^23 molecules/mole)/ (8.314 Pa-m3/mol-K)(273.15K)
molecules/V = 7.339x10^27 molecules/m^3 - Final answer
