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Measuring Gases

This article explains atmospheric pressure, its relationship with altitude and water vapor, measurement methods using barometers, conversion factors, and the behavior of enclosed gases at standard temperature and pressure.

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Measuring Gases

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  1. Measuring Gases College Chemistry

  2. Atmospheric Pressure • The atmosphere around us exerts pressure on us – equal to the pressure we exert on it • If the atmosphere exerts more pressure  we would be crushed (scuba diving) • If the atmosphere exerts less pressure  we would explode (spray can in the summer)

  3. Atmospheric Pressure • Atmospheric pressure – pressure exerted by the air in the atmosphere • Result of the fact that air has mass and is attracted by Earth’s gravity  gravity produces a force on us • Depends on altitude

  4. Atmospheric Pressure • Varies with altitude – lower the altitude, the longer and heavier the column of air above the area of earth is • Atmospheric pressure also varies with amount of water in the air • The heavier the air is  the LESS water is in the air • Water vapor is NOT heavier than air!!!!!!!!

  5. Atmospheric Pressure • Barometer – measures the pressure of the air • Low pressure – bad weather (rain – lots of water) • High pressure – good weather

  6. Atmospheric Pressure • Usually in a barometer there is a column of mercury  measure pressure in mmHg

  7. Units of Pressure • Right now you have 1 atmosphere (atm) of pressure on you • Pressure is also measured in Pascals, kilopascals, mmHg, torr, and bar • Learn and memorize these conversion factors!!!

  8. Conversion Factors • 1 atm = 101,325 Pa (Pascals) • 1 atm = 101.3 kPa (kilopascals) • 1 atm = 760 mmHg = 760 torr • 1 atm = 14.70 lb/in2 (psi) • 1 bar = 0.9869 atm

  9. Example • The column of mercury in a barometer is 745 mm above the bottom. What is the atmospheric pressure in Pascals? • Use dimensional analysis • 745 mmHg x (101,325 Pa/760 mmHg) = 99,300 Pa

  10. Another Example • The air pressure inside the cabin of an airplane is 8.3 lb/in2. What is the pressure in atmospheric units? • 8.3 lb/in2 x (1 atm / 14.70 lb/in2) = 0.56 atm

  11. Enclosed Gases • If a gas is open to the atmosphere, some of the gas will escape • Eventually the inside gas and the atmosphere will reach equilibrium and have the same value • In an enclosed gas, the pressure inside can be different than atmospheric pressure

  12. Enclosed Gases • Enclosed gases are measured with a manometer • One end opens into the container with the gas pressure to be measured, the other is open to the surrounding atmosphere

  13. STP • Behavior of gas is STRONGLY dependent on pressure and temperature • Because of this, we usually talk about gases at STP – standard temperature and pressure • 273 K (freezing point of water, 0⁰C) and 1 atmosphere

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