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 = kPa (kilopascals)
1 atm = 760 mmHg = 760 torr
1 atm = lb/in2 (psi)
1 bar = 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 / 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