The Ideal Gas Law states that pressure (P) × volume (V) is equal to the # of moles (n) of the gas × a constant (R) × temperature (T), such that the equation is:
PV = nRT
At standard temp and pressure (STP), the T is 0°C or 273.15K, the P is 1 atm or 760 torr, and the R constant is 0.0821. Therefore the equation, solved for V becomes: V = nRT/P, or V = n(0.0821)(273)/1, so that it reduces to V = 22.4 Liters, when n = 1 mole.
So the V of any gas at STP is 22.4 L / mole