Molar volume of an ideal gas at STP (0 °C, 100 kPa)

One mole of any ideal gas occupies RT/p, so the number is meaningless until you state T and p. At the modern IUPAC definition of STP, 273.15 K and 100 kPa (1 bar) exactly, that volume is 22.710 954 64 L/mol, or 0.02271095464 m³/mol.

The famous 22.4 L/mol belongs to the older standard pressure of 1 atm = 101.325 kPa; IUPAC moved standard pressure to 1 bar in 1982 to match the standard state used in thermodynamic tables, and the molar volume moved with it by 1.3 %. Textbooks and exam boards did not all follow at the same pace, which is why both numbers are still in circulation. Real gases deviate: at 0 °C and 1 bar, CO₂ occupies about 0.5 % less than the ideal value and ammonia about 1.5 % less.