Why is STP mol/liter so specific? [duplicate]

Question

How do we know that one mole of an ideal gas occupies (exactly) 22.4 Liters? Why is it so precise a value? Many other constants/conversions in science have multiple decimal places, so why is this conversion terminated so soon?

Yes, I have googled it, and searched it up on StackExchange, and flipped though my chemistry textbook. I just want a nice clear explanation for the exact value of the the mol/liter conversion.

Finally, please don't use the ideal gas constant $\ce{R=8.314 J/K\cdot mol}$ to justify your answer, because we get $R$ by using this conversion fact.

Thank you for any information or insights!

EDIT: This has been suggested as a duplicate of What volume does one mole of an ideal gas occupy?, which does not answer my question because it derives the liter/mol conversion from the ideal gas constant.

Answer 1

How do we know that one mole of an ideal gas occupies (exactly) 22.4 litres?

It doesn't.

The current recommended value for the molar volume of an ideal gas at a temperature of $T=273.15\ \mathrm K$ and a pressure of $101.325\ \mathrm{kPa}$ is $V_\mathrm m=22.413962(13)\times10^{-3}\ \mathrm{m^3\ mol^{-1}}$ (source). Note the given uncertainty; i.e. it is an experimental value that is not exact.

Also note that these conditions do not correspond to STP. According to current IUPAC recommendations, STP corresponds to a temperature of $T=273.15\ \mathrm K$ and a pressure of $100\ \mathrm{kPa}=1\ \mathrm{bar}$. At this state, the molar volume of an ideal gas actually is $V_\mathrm m=22.710947(13)\times10^{-3}\ \mathrm{m^3\ mol^{-1}}$ (source).