Anonymous asked in Science & MathematicsPhysics · 9 years ago

Why is the pressure at SATP, 100 kPa, less than STP, 101.325 kPa?

Especially when the temperature is higher at SATP...shouldn't that mean the pressure at 25 Celsius should be slightly more than 0 Celsius?

SATP - 25 Celsius and 100 kPa

STP - 0 Celsius and 101.325 kPa


This is what it says in the textbook and various sites I checked, so I don't have it mixed up, I'm sure (even though it should be the other way around).

Update 2:

Sorry anyone, I didn't mean to sound "righteous" or anything with my previous "additional detail". My textbook (from school) says exactly what I wrote down. The textbook is pretty old (2002), so I'm sure IUPAC has changed things around since then. All I wanted to find out was why they are that way, not if I was right or wrong. I wanted to have my textbook explained, not an explanation of how I/my textbook was wrong. In my homework, I have to use those values for STP and SATP in my calculations.

3 Answers

  • 9 years ago
    Favorite Answer

    STP is atmospheric pressure and zero celsius temperature. The reason why we use it as standard (even though it is a bit frigid), is that it is easy to experimentally produce a zero C temperature, since all that must be ensured is two-phase coexistence of water and ice.

    SATP is adjusted to a more comfortable temperature, more common in a laboratory. 25 C is used because it is an easy number to remember, and because it is the datum of most JANF tables of chemistry data. The 100 kPa as opposed to 101.325 kPa is used just to avoid the excessive numbers. AND because, 100 kPa is more common to more locations than the sea level 101.325 kPa. 100 kPa is approximately 100 meters of elevation above sea level, and an easy number to remember. 100 kPa is referred to as the bar, rather than as the atmosphere. I suppose you can consider 1 bar to be 1 "metric atmosphere".

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  • D g
    Lv 7
    9 years ago

    AS YOU can see you have the values MIXED up

    its SATP and STP are actually the same... they are just different names for the same thing as quoted on Wikipedia..

    your values are reversed..

    0 degs has a 100kPa value

    and 20 degs has the 101.325 kPa value.

    Im sorry that is a fact

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  • Anonymous
    9 years ago

    you have it mixed up:

    STP (Standard Temp and Pressure) is 0 celsius and 100 kPa

    SATP (Standard ambient temp and pressure) is around room temp and its 25 celsius and 101.325 kPa

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