Chapter 11 · Gases
11.3Pressure: The Result of Constant Molecular Collisions
10 min · two checks
Predict
Which way do you convert 2.00 atm to kPa?
The idea
- Define pressure as force per area.
- Convert among atm, kPa, and mmHg.
Pressure is force divided by area. The SI unit is the pascal, 1 N/m², and chemists usually speak in kilopascals because a pascal is a small pressure. Standard atmospheric pressure is 1 atm = 101.325 kPa = 760 mmHg = 760 torr. A bar is 100 kPa, close to 1 atm but not identical. When a problem says “standard pressure” without detail, check which convention the course is using; many intro problems still take 1 atm.
Manometers and barometers compare a gas pressure with a liquid column or with the atmosphere. You rarely need the instrument design to do the arithmetic. You do need to convert units before a gas law, because R, the gas constant, is married to a specific pressure unit. Mixing atm with a kPa version of R is a silent factor-of-about-100 error.
Keep these
- 1 atm = 101.325 kPa = 760 mmHg.
- 1 bar = 100 kPa, slightly less than 1 atm.
- Match the pressure unit to the gas constant you choose.
Worked path
Convert 95.0 kPa to atmospheres.
Observe
95.0 kPa has three significant figures. Want atm.