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Understanding Pressure Units in Chemistry

Apr 3, 2025

Pressure Units in Chemistry

Common Pressure Units

  • Millimeters of Mercury (mmHg):

    • First developed method for measuring atmospheric pressure using a barometer/manometer.
    • Measures the distance the mercury travels to compare pressures.
    • Also known as "torr" after Torricelli.
  • Atmosphere (atm):

    • Commonly used unit.
    • 1 atm corresponds to standard sea level pressure and temperature of 0°C (STP).
    • Conversion to Kelvin (K): add 273 to Celsius.
  • Pascal (Pa):

    • Symbol: Pa; usually use kilopascals (kPa) due to large numbers.
    • 1 Pa = 1 Newton/m².
    • Conversion: 101.325 kPa = 1 atm = 760 mmHg = 760 torr.
  • Pounds per Square Inch (psi):

    • Not typically used in science (American unit).
    • Used in diving and for tire pressures.
    • Conversion: 1 atm = 760 torr = 760 mmHg = 101.325 kPa = 14.7 psi.

Important Conversions and Constants

  • Standard Temperature and Pressure (STP):

    • Defined as 0°C and 1 atm.
    • Kelvin conversion: 0°C = 273 K.
    • Key constants:
      • Molar volume at STP = 22.4 L
      • Avogadro's number = 6.022 x 10²³
  • Conversions:

    • mmHg to torr: 1:1 ratio.
    • atm to mmHg: 1 atm = 760 mmHg.
    • kPa to atm: 101.325 kPa = 1 atm.
    • psi to atm: 14.7 psi = 1 atm.

Notes on Usage

  • Kilopascals preferred over pascals for ease of use.
  • PSI readings common in practical applications like diving and tire pressure measurement.
  • Scientific calculations typically avoid American units like psi.

Additional Notes:

  • Stay tuned for notebook examples on using these conversions in practice.
  • Remember: Kelvin is a measure of energy, not temperature; no negative Kelvin values.