The scientific units of pressure can be determined from its definition:

A force applied to surface with an area A will have will result in a pressure P as defined above. Force has the units "mass length/time^2" and area has the units length^2. Inserting this into the equation above results in the units of pressure as

A pascal is the SI unit of pressure.

Consider gas molecules in a rectangular box. Every time a molecule collides with a wall of the box the collision results in a force on the box. These forces combine and result in the pressure of the gas.

Units of Pressure

As shown above the SI unit of pressure is the pascal (Pa). However, the pascal is not the most convenient unit of pressure. The table below lists some more common units of pressure.

 Pressure Unit  Abbrev.  Conversion
 Pascal  Pa  ---
 mm of mercury  mmHg  ---
 atmosphere  atm
760 mmHg
=101,325 Pa
 bar  bar  100,000 Pa
 Torr  Torr  mmHg
 pound/sq inch  lb/in^2  51.7 mmHg


Most calculations in chemistry involving gases use the pascal and the atmosphere. However, coversions between the various units are straighforward. Example How many pascals is 500 mmHg? Using the table on the left we find:

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C101 Class Notes
Prof. N. De Leon