Measuring Measuring GasesGases

College Chemistry College Chemistry

Atmospheric PressureAtmospheric 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 • 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

Atmospheric PressureAtmospheric 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 PressureAtmospheric Pressure• Barometer – measures the pressure

of the air– Low pressure – bad weather (rain – lots

of water)– High pressure – good weather

Atmospheric PressureAtmospheric Pressure• Usually in a barometer there is a

column of mercury measure pressure in mmHg

Units of PressureUnits 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!!!

Conversion FactorsConversion 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

ExampleExample• 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

Another ExampleAnother 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

Enclosed GasesEnclosed 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 GasesEnclosed 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

STPSTP• 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