Try not to get confused (as I did) between the standard temperature and pressure for
the Gas Laws
| STP | |
| Temperature = | 0ºC |
| Before 1990: Pressure = | ì101.325 kPa í1atm î760 mmHg |
| Since 1990: Pressure = | ì100 kPa (1 bar) í0.98692 atm î750 mmHg |
and the thermodynamic standard state conditions
| Thermodynamic Standard State | |
| Temperature (may be) = | 25ºC (298.15 K) |
| Pressure Pº= | ì100 kPa (1 bar) |
It's been some time now (since 1983) that the thermodynamic standard pressure has been 100 kPa (alias 1 bar). An older non-SI unit of pressure used to measure pressure and a different value for standard pressure was defined in that system. The old Standard pressure was 1 atmosphere (1 atm) where one atmosphere is taken to be the average atmospheric pressure at sea-level at about 45º longitude on the surface of the earth. While this notion of how to determine the standard pressure has some endearing charm to it, it does not result in a number that is easy to use calculationally. It is far easier to stick with the SI units of Pa. It just turns out that 100 kPa is very nearly the same pressure as 1 atmosphere.
100 kPa = 0.98692 atm = 1 bar.
This is the new standard pressure for both Standard state calculations (thermodynamics) and STP (gas law calculations)