1. Ch 12 Gases
Though the chemical behavior of gases differ, all gases have very similar physical behavior
Basic properties of gases

2. The physical behavior of gases can be described completely by four variables:

3. Gas Pressure Pressure = total force applied to a certain area
larger force = larger pressure
smaller area = larger pressure
Gas pressure caused by gas molecules colliding with surface of their container
More forceful collisions or more frequent collisions mean higher gas pressure 3

4. Air Pressure Air is a mixture of gases (O2, N2, CO2, etc.)
Constantly present when air is present
Decreases with altitude
Due to less overlying air pressing down
Varies with weather conditions
Measured using a barometer (fig 12.2)
Column of mercury supported by air pressure
Longer mercury column supported = higher pressure
Force of the air on the surface of the mercury balanced by the pull of gravity on the column of mercury 4

5. Units of Gas Pressure mm Hg
units for the height of a column of mercury the gas pressure can support.
Standard atmosphere (atm)
equals mm Hg (at sea level)
torr
named after Evangelista Torricelli who invented the barometer in 1643
760.0 mm Hg = torr
pascal (Pa) – named for Blaise Pascal, 17th c. scientist
pounds per square inch (psi, lbs./in2)
Conversions:
1.0 atm = mm Hg = in Hg = torr = 101,325 Pa = kPa = psi 6

6. 12.2 Pressure and Volume: Boyle’s Law
Robt. Boyle, 17th c.
Pressure is inversely proportional to Volume
constant T and amount of gas
graph P vs V is curve (fig 12.5)
as P increases, V decreases by the same factor
P x V = constant (table 12.1 – sample of Boyle’s observations)
Therefore, Boyle’s Law is expressed as:
PV = k 7

7. According to Boyle’s Law, if the initial volume and pressure is known, the new volume or pressure can be predicted.
P1 x V1 = P2 x V2
Example: Calculating new volume
A sample of gas has a volume of 1.50 L at 56.0 torr. What will be t the volume if the pressure is increased to torr?

8. Example: Calculating new pressure
A sample of a gas has a volume of 1.50 L at a pressure of 56.0
torr. What is the pressure when the volume decreases to L?

9. 12.3 Volume and Temperature: Charles’ Law12

10. Absolute Zero
The theoretical temperature at which a gas would have zero volume and no pressure
calculated by extrapolation (see fig 12.7)
never actually attained, although have come close
0 (zero) K = °C = -459 °F
Kelvin temp = Celsius temp + 273
All gas law problems use Kelvin temperature scale! 11

11. Calculating Volume change:
Calculating Temperature change:

12. 12.4 Avogadro’s Law

13. Calculating volume change
Calculating mole change

14. Gay-Lussac’s Law

15. 12.5 Ideal Gas Law
The relationships between the P, T, V and number of moles (n) of a gas can be combined together into one general equation called the Ideal Gas Law:
R is the universal gas constant – has the value L atm/K mol
Use the ideal gas law when three of the four properties of the gas are known – then solve for the remaining property
Though no ideal gas actually exists, most simple gases show nearly ideal behavior at ordinary temperatures and pressures
PV = nRT 14

16. Solving for P, V, T, n

17. Combined Gas Law:
A combination of Boyle’sLaw, Charles’ Law, and Gay-Lussac’s Law.
Use in place of Ideal Gas Law when moles remains constant.
Compares the same substance under two different sets of conditions.
Rearrange the equation to solve for V2, P2, or T2 under changing conditions. 15

18. Calculating for P, V, T changes

19. Use PV = nRT to solve any gas problem