Lecture on Gas Pressure

Introduction to Gas Pressure

• Learning objectives provided at the beginning, review them.
• Gas pressure introduced via atmospheric pressure.
• Atmospheric pressure: Force exerted by a column of air on one square inch of space (~14.7 lbs/sq. inch = 1 atmosphere).

Units of Pressure

• General definition: Pressure (P) = Force/Area
• Pressure directly proportional to force, inversely to area.
• Common units:
  • Pascal (Pa): SI unit, defined as Newton per meter squared.
  • Kilopascal (kPa): 1 kPa = 1000 Pa.
  • Pounds per square inch (psi): 14.7 psi = 1 atmosphere at sea level.
  • Atmosphere (atm): 1 atm = 101,325 Pa = 760 torr.
  • Bar and Millibar: 1 bar = 100,000 Pa.
  • Inches of Mercury (inHg): Used in barometers and manometers, 1 inHg = 3,386 Pa.
  • Torr and Millimeters of Mercury (mmHg): 1 atm = 760 torr = 760 mmHg.

Measuring Pressure

Barometers

• Used to measure atmospheric pressure.
• Liquid Mercury vs. Water: Mercury preferred due to higher density, reducing tube size needed for displacement measurement.
• Mechanism: Atmospheric changes displace liquid in the tube, measures in inHg or mmHg.

Manometers

• Measures pressure of gas samples (not atmospheric pressure).
• Closed End Manometer: Measures pressure without atmospheric influence.
  • Formula: Pressure = Height x Density (ρ) x Gravity (g).
• Open End Manometer: Accounts for atmospheric pressure.
  • Case 1: Sample pressure < atmospheric pressure (negative displacement).
  • Case 2: Sample pressure = atmospheric pressure (no displacement).
  • Case 3: Sample pressure > atmospheric pressure (positive displacement).
• Gas Pressure Calculation:
  • Pressure in open end manometer = Atmospheric pressure + Height x Density x Gravity.

Example Problems

Closed End Manometer

• Given: 26.4 cm of Hg displacement.
• Convert to torr:
  • 26.4 cmHg = 264 torr (using 1 cm = 10 mmHg, then 1 mmHg = 1 torr).

Open End Manometer

• Given displacement indicates pressure > 1 atmosphere.
• Calculate pressure in atmospheres:
  • 13.7 cmHg converted to 137 mmHg.
  • Gas pressure = Atmospheric pressure + 137 mmHg = 897 mmHg.
  • Convert to atm: 897 mmHg / 760 mmHg = 1.18 atm.