Measuring Gases College Chemistry Atmospheric Pressure The atmosphere

Measuring Gases College Chemistry

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)

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 produces a force on us • Depends on altitude

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!!!!

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

Atmospheric Pressure • Usually in a barometer there is a column of mercury measure pressure in mm. Hg

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

Conversion Factors • 1 • 1 • 1 atm = 101, 325 Pa (Pascals) atm = 101. 3 k. Pa (kilopascals) atm = 760 mm. Hg = 760 torr atm = 14. 70 lb/in 2 (psi) bar = 0. 9869 atm

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 mm. Hg x (101, 325 Pa/760 mm. Hg) = 99, 300 Pa

Another Example • The air pressure inside the cabin of an airplane is 8. 3 lb/in 2. What is the pressure in atmospheric units? – 8. 3 lb/in 2 x (1 atm / 14. 70 lb/in 2) = 0. 56 atm

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

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

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