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.