Chemical Units

Conventions

There are two different conventions for representing data, depending on the situation the data will be used in. The second method is recommended by IUPAC for use in labelled columns or graphs.

Tabular

Write the physical quantity (e.g. 320) followed by the unit in parentheses (e.g. "(g)").

Examples:

  • 500 (g) - Mass
  • 6.78 (Pa) - Pressure
  • 2.03 (kJ mol-1) - Delta H (Change in entropy)
  • 23489 (10-5L mol-1 s-1) - k (Reaction rate)

Graphical

Divide the physical quantity by the unit, creating a 'pure number' - one that is easily graphable.

Miscellaneous Unit Facts

Pressure Units

  • Pressure = Force per Area
  • Measured in Pascals (Pa)
    • 1 Pa = 1 N m-2)
  • KPa, MPa or bar are more common than Pa (103 Pa, 106 Pa and 105 Pa, respectively).
  • Pressure used in defining normal boiling temperature of a liquid, or melting point of a solid = 1 Pressure of atm (atmosphere)
  • Atmosphere is measured in (atm)
  • 1 atm = 1.013 * 1 bar

mmHG/Torr

  • Pressure used to be measured with a bowl of mercury and a tube in it. If pressure changed mercury flowed up the tube, and hence was measured in millimetres.
  • Torr was meant to be the same as mmHG, but with redefinitions is different by less than 0.000015%
  • 1 mmHG = 1.33 Pa
  • 1 mmHG = 1 atm/760

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