What are the standard temperature (in Kelvin) and pressure (in atm.) conditions for gas measurements?
Many online sources actually get this one wrong.
STP is set by the IUPAC as 0°C and 100kPa. Many old books and sources online will say that standard pressure is 101.325kPa (the old IUPAC standard which was changed in 1982), but I defer to the current IUPAC of 100kPa. See this link from Wikipedia for further discussion.
0°C is the same as 273.15K.
100kPa converts to 0.986923267 atm
Hope this helps!