Why is STP mol/liter so specific? 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.
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).