Gases expand and contract (or change volume) in temperature and pressure.
STP= 1 atmosphere (atm) of pressure, and temperature at 0 degrees Celsius of 273.15K
At STP, there are 22.4 L per 1 mole
Therefore we have this mole conversion:
22.4L of gas 1 mol of gas
1 mol of gas OR 22.4L of gas
