1. Chapter 4B: Gas Laws p
Questions p (47-71)

2. Gas Laws
There are a few laws that all gases must follow based on their Pressure, Volume, and Temperature.
Do you have to memorize them?
NO! Table T has one and you can modify it for the other two.
Boyle’s Law
Charles’ Law
Combined Gas Law

3. Boyle’s Law
P1V1 = P2V2
So, you need Pressure and Volume
Volume is expressed as mL or L
Pressure is expressed in atm or kPa
At constant temperature, demonstrates the relationship between pressure and volume
Indirect relationship
Pressure increases, volume decreases
Pressure decreases, volume increases

4. Boyle’s Law

5. Example 1:
A gas has a volume of 50mL at a pressure of 1atm. If the volume is increased to 100mL, and the temperature is constant, what is the new pressure?

6. Example 2:
A gas occupies 25mL at 2atm. What will be the volume of this gas if the pressure becomes 4atm and the temperature remains constant?

7. Charles’ Law
V1/T1 = V2/T2
So, you need Volume and Temperature
Volume is expressed in mL or L
Temperature MUST be expressed in KELVIN!
So, first, you have to convert temperature to the Kelvin scale- it avoids using a negative number. Table T
At constant pressure, demonstrates the relationship between volume and temperature
Direct relationship
Temperature increases, volume increases
Temperature decreases, volume decreases

8. Charles’ Law

9. Example 3:
A gas has a volume of 100. mL at a temperature of 200. K. Calculate the new volume if the temperature was increased to 400. K and the pressure remains constant.

10. Example 4:
At -73oC, the volume of a gas is 100mL. The temperature is raised to 27oC, and the pressure remains constant. What is the new volume?

11. Combined Gas Law (P1V1)/T1 = (P2V2)/T2
This law combines both Boyle’s Law and Charles’ Law into one law.
This is the formula that is given to you in your Reference Tables
Table T
Fill in what is given to you and leave the rest alone.
Just ignore it!
Only use Kelvin temperatures

12. Example 5:
A gas has a volume of 15.0L at a temperature of 300.K and a pressure of 4.00atm. What will be the new volume when the temperature is changed to 400K and the pressure is changed to 2.00atm?

13. Example 6:
The volume of a sample of a gas at 273oC is 200.0L. If the volume is decreased to 100.0L at constant pressure, what will be the new temperature of the gas?

14. STP
Whenever a problem says that the gas is at STP, (according to Table A of your Reference Tables) that means:
Standard Pressure
101.3 kPa
1 atm
Standard Temperature
273 K
0oC
Which one you use will depend upon the units that are given.

15. Kinetic Molecular Theory of Gases
Gas laws describe HOW gases behave, but not WHY they behave the way that they do
The Kinetic Molecular Theory (KMT) is a theory that is used to explain the behavior of gases
Also called Ideal Gas Laws

16. Kinetic Molecular Theory (aka Ideal Gas Laws)
Gases contain particles that are in constant, random, straight-line motion.
Gas particles collide with each other and with the walls of the container. These collisions may result in a transfer of energy among the particles, but there is no net loss of energy as the result of these collisions. The total energy remains constant.
Gas particles are separated by relatively great distances (for their size). Because of this, the volume occupied by the particles themselves is negligible and need not be accounted for. In other words, it is mostly empty space.
Gas particles do not attract each other.

17. Ideal verses Real Gases
KMT explains the behavior of gases by using a model gas called an “ideal” gas
A gas is said to be “ideal” if it behaves exactly as predicted
When gas laws are used to solve problems involving “real” gases, the answers obtained often do not exactly match the results obtained in the lab.
Its theoretical
Ideal gases do not exactly match the behavior of real gases
This is because 2 of the assumptions made by KMT are not exactly correct.

18. Ideal Gas Law Discrepancies- #3
Gas particles do not occupy volume.
Although gas particles themselves occupy a small volume of space under normal conditions, as pressure increases the volume occupied by the particles can no longer be ignored.
At high pressures, the increased concentration of particles leads to more frequent collisions and far greater chances of combining

19. Ideal Gas Law Discrepancies- #4
Gas particles do not attract one another.
In most cases, the attractive forces between gas particles are so small that they can be disregarded.
However, in some extreme cases, these small forces become important.
ie- Water molecules in the atmosphere attract each other when the temperature becomes cold enough. The water molecules combine to form snow or rain.
Real gas particles have weak intermolecular forces of attraction.
Anything that has mass has an attraction!

20. What is “ideal?”
A gas is said to be “ideal” if it behaves EXACTLY as predicted
No gas is truly “ideal”
Hydrogen (H) and Helium (He) are nearly ideal because they are so small
MEMORIZE THIS!
Gases are most ideal at low pressures and at high temperatures

21. Ideal Gases
A real gas behaves most like an ideal gas at a LOW pressure and a HIGH temperature.
Why?
Low Pressure: gas particles are TOO FAR apart for weak intermolecular forces to form
High Temperature: gas particles are moving TOO FAST to have any intermolecular forces of attraction

22. Avogadro’s Number (RECAP)
Collective noun
ie- Dozen=12, Gross=144, etc.
Equal volumes of all gases under the same conditions of temperature and pressure have equal numbers of molecules
One mole (mol) has 6.02 x 1023 particles/mol.
12L of Nitrogen gas at STP would contain the same number of molecules as 12L of Oxygen at STP
One mole of gas occupies a volume of 22.4L at STP
MEMORIZE THIS!!! It is NOT given to you in your Reference Tables!

23. Example 7:

24. Example 8:
At STP, equal volumes of N2(g) and CO2(g) contain equal numbers of
Protons
Electrons
Atoms
Molecules

25. Example 9:
How many molecules of H2(g) are in 44.8L of the gas?

26. Open your workbook to p. 69 (47-66)F f