1. Section 1: Combined Gas Law
Unit 6: Gases
Section 1: Combined Gas Law

2. Overview
Gases provide the breath of life, inflate tires, power hot-air balloons, dissolve in our blood, heat our homes, inflate air bags, etc.
Gases span from necessity to usefulness to extreme toxicity

3. Introduction Looks at how molecules of a gas behave
Kinetic molecular theory: the model for the action of gas molecules in a confined space
Kinetic means moving, so the theory focuses on how gas molecules behave as a result of their motion
Pressure, temperature, and volume are all factors that affect the behavior of gases

4. Pressure
When the molecules of a gas in a container collide with the inside surfaces of the container, causing their speed and mass to push against the walls
Atmospheric Pressure
Has weight and exerts pressure on everything
We don’t notice this pressure much because it’s the same inside and outside our homes, the same inside and outside our body, etc.
We notice the pressure change when riding on an airplane
Also known as barometric pressure
Units of pressure
1 atm = 14.7 inches = 760 mm = 760 torr = 101,325 pascals

5. Temperature
Measure of the speed at which molecules and atoms are moving
The higher the temperature, the faster the speed
Units of temperature
Fahrenheit (ºF), Celsius (ºC), and Kelvin (K)
Gases uses the Kelvin temperature scale, since it’s based on the idea of absolute zero
Absolute zero (0 K) is where all movement stops
Temperature conversion
To change from ºC to K, add 273
To change from K to ºC, subtract 273

6. Volume Describes an amount of space
Volume units include gallons, quarts, liters, cm3, and ounces
Can be either fixed or elastic
Fixed: volume is contained and has a definite shape
Drink can, box, Tupperware
Elastic: volume can change
Balloon, raft, basketball

7. Mathematical Approach to Gases
Majority of gas behavior can be described mathematically by two equations: the combined gas law and the ideal gas law
Combined gas law is used for situations involving change
P1V1 = P2V2
T1 T2
P = pressure
V = volume
T = temperature (in kelvins)

8. Combined Gas Law
This equation can be used to consider just pressure and volume, just pressure and temperature, just volume and temperature, or all three, without having to remember different equations for each situation
P1V1 = P2V2
P1 = P2
T1 T2
V1 = V2
T T2
T T2

9. Boyle’s Law
Named after Robert Boyle, a 17th century Irish scientist who studied the relationship between pressure and volume in gases, keeping temperature constant
Boyle’s Law states that volume and pressure are inversely proportional
As volume increases, pressure decreases
P1V1 = P2V2
Example: How many liters in size will a balloon become at a pressure of 700 mm if it has a volume of 4 liters at a pressure of 600 mm?
(600 mm)(4 L) = (700 mm)(V2)
(600)(4) = V2
700
3.43 L = V2

10. Charles’ Law
Named after Jacques Charles, an 18th century French physicist who studied the relationship between temperature and volume with respect to gases, keeping pressure constant
Charles’ Law states that temperature and volume are directly proportional
As temperature increases, volume increases
V1 = V2
T T2
Example: What is the volume of a helium-filled balloon at 200 K if it has a volume of 8 L at 100 K?
8 L = V2
100 K K
(8)(200) = V2(100)
1,600 = 100 V2
16 L = V2

11. Gay-Lussac’s Law
Gay-Lussac understood what happens to the pressure of an enclosed gas when heat is applied
Knew that intense pressure builds up as the gas molecules move faster and strike the walls of the container more often and with more force
Gay-Lussac’s Law states that temperature and pressure are directly proportional
As temperature increases, pressure increases
P1 = P2
T1 T2
Example: Calculate the pressure that results when the temperature becomes 100ºC if the pressure at 25ºC is 750 torr.
750 = P Convert ºC to K first!!!
P2 = (750)(373)
298
P2 = torr

12. Combination Theory
Most real applications of gas theory involve varying the temperature, pressure, and volume
P1V1 = P2V2
T T2
Example: What pressure is expected to develop at a volume of 10 L and a temperature of 25ºC if the volume is 5 L, the temperature is 50ºC, and the pressure is 30 inches of mercury?
T T2
(30 inches)(5 L) = (P2)(10 L)
(30)(5) = (10)(P2)
(30)(5)(298) = (10)(P2)(323)
(30)(5)(298) = P2
(10)(323)
13.84 inches = P2