1. Gases and Pressure
Chapter 11
Section 1

2. Vocabulary Pressure: the force per unit area on a surface
Atmospheric pressure: the force per unit area exerted against a surface by the weight of the air above that surface.

3. Measuring Pressure
Barometer: device used to measure atmospheric pressure
First introduced by Evangelista Torricelli during the early 1600s
Discovered that atmospheric pressure supports a column of mercury about 760 mm above the surface of Hg in a dish

5. Units of Pressure There are several units for pressure.
mm Hg, also called torr in honor of Torricelli
Average atmospheric pressure at sea level at 0°C is 760 mm Hg, or 760 torr
Atmospheres: one atm is defined as being exactly equivalent to 760 mm Hg

6. More about Units
The SI unit of measurement for pressure is the pascal.
One pascal (Pa) is the pressure exerted by a force of one newton acting on an area of one square meter.
Usually expressed as kPa
1 atm = kPa

7. STP STP = standard temperature and pressure
STP has conditions of exactly 0°C and 1 atm of pressure.

8. Dalton’s Law of Partial Pressures
The pressure of each individual gas in a mixture of gases is called the partial pressure of that gas.
Dalton’s law states that the total pressure of a gas mixture is the sum of the partial pressures of the component gases.
PT = P1 + P2 + P3 + …

9. Problem 1
A mixture of O2, CO2, and N2 has a total pressure of 0.97 atm. What is the partial pressure of O2 if the partial pressure of CO2 is 0.70 atm and the partial pressure of N2 is 0.12 atm?
PT = Poxygen + Pcarbon dioxide + Pnitrogen
Poxygen = PT – (Pcarbon dioxide + Pnitrogen)
Poxygen = 0.97 atm – (0.70 atm atm)
Poxygen = 0.15 atm

10. Problem 2
Find the total pressure for a mixture that contains four gases with partial pressure of 5.00 kPa, 4.56 kPa, 3.02 kPa, and kPa.
PT = P1 + P2 + P3 + P4
PT = ( ) kPa
PT = kPa

11. Water Displacement
Gas can be collected by water displacement, but some water vapor will be present.
The total pressure will be the sum of the partial pressures of the gas collected and partial pressure of water vapor.
The partial pressure of water vapor is found on a table (dependent on temperature).

12. Gas Collected by Water Displacement Figure 5- page 366
Pgas = Patm – Pwater vapor

14. Problem 3
O2(g),from the decomposition of KClO3, was collected by water displacement. The barometric (atmospheric) pressure and the temperature during the experiment were 97.5 kPa and 20.0° C. What was the partial pressure of the O2 collected?

15. Solution PT = Patm = 97.5 kPa Pwater vapor = 2.34 kPa
Unknown = Poxygen gas
Poxygen gas = PT – Pwater vapor
Poxygen gas = 97.5 kPa – 2.34 kPa
Poxygen gas = 95.2 kPa