We sometimes like to think about things "in reference" to certain standards. In the world of ideal gases, we often refer to standard temperature and pressure or STP. STP is simply a temperature of 0 °C (273.15 K) and 1 atm pressure (the old standard).
If you plug those numbers into the ideal gas law and use exactly 1 mole of gas you will calculate the volume of an ideal gas at STP. That volume is 22.4 L. Lot of folks (scientists, teachers, and students) like to memorize that number because it comes in handy many times. There IS a bit of a trick area though.
STP is only 273.15 K when calling the temperature "standard" temperature for a gas. This of course is NOT "room temperature" which is accepted to be 25 °C or 298.15 K. Always know which standard is being referenced. I'll always try to say "room" temperature when I want you to use 25 °C. You CAN have lots of standards and you just have to know which one is "in play" for problem solving. Room temperature is sometimes distinguished as "ambient" temperature. Also the new standard for pressure is actually the bar. So lets point this out with two designations:
STP the old standard temperature and pressure: 273.15 K and 1 atm
SATP the new standard ambient temperature and pressure: 298.15 K and 1 bar
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