STP or Standard Pressure and Temperature is widely used when the subject is gases. The used standard temperature is 273 K or 0°C and the standard temperature used is 1 atm. At these conditions 1 mol will be equal to 22.4 L of volume, therefore if we have 11.2 L of Argon:
22.4 L = 1 mol
11.2 L = x moles of Argon
22.4x = 11.2 L
x = 0.500 moles of Ar will be present in 11.2 L of volume
