What is the volume of 1 mole of any gas at STP?

In reality the 22.4 liters per mole is only an approximation for many gases. Ideal gases actually do not exists though inert gases behave very close to ideal gas laws. The interactions between molecules do affect the behavior of the gases making small changes to the ideal value of 22.4 liters for an ideal gas.

The IUPAC defines STP as a temperature of 0 °C and a pressure of 1 bar.

We can use the Ideal Gas Law to calculate the molar volume of an ideal gas.

#color(blue)(bar(ul(|color(white)(a/a) PV=nRTcolor(white)(a/a)|)))" "#

We can rearrange this to give

#V = (nRT)/P#

∴ #V = (1 color(red)(cancel(color(black)("mol"))) × "0.083 14" color(red)(cancel(color(black)("bar")))·"L"·color(red)(cancel(color(black)("K"^"-1""mol"^"-1"))) × 273.15 color(red)(cancel(color(black)("K"))))/(1 color(red)(cancel(color(black)("bar")))) = "22.71 L"#

In practice, NO real gas is an ideal gas.

Even helium, which should be closest to an ideal gas, has a molar volume at STP of 22.73 L.

In most cases, however, the deviations are so small under "ordinary" conditions that we can treat the gases as if they were ideal.