1. Section 3: Phase Changes & Behavior of Gases
Unit 3: Weather
Section 3: Phase Changes & Behavior of Gases

2. Lesson 16: STP – The Mole and Avogadro’s law

3. ChemCatalyst
There are two balloons. One is filled with helium, He, and the other with carbon dioxide, CO2.
Describe what happens when the balloons are released.
For the two balloons, state whether these properties are the same or different, and explain why:
Pressure, P
Temperature, T
Volume, V
Mass, m
Number of molecules, n
Density, m/V

4. Key Question:
How do chemists keep track of the number of gas particles?

5. Objectives: define a mole explain Avogadro’s law
define standard temperature and pressure

6. Dry Ice Setup: Do not touch the dry ice with bare hands.
Use tongs or gloves.
One student from each group should get a polystyrene foam cup filled with 2-3 g of dry ice from the teacher and a balloon.
Find the mass of the cup and the dry ice.
Quickly pour the dry ice into the balloon and tie it tightly. Be careful to keep air out.
Mass the empty cup. Subtract this weight from the mass of the cup containing the dry ice in order to determine the mass of the dry ice.

7. Introduction – the Mole
Chemists use a unit called a mole to describe the number of gas particles in a sample.
1 mole = 602,000,000,000,000,000,000,000 or
6.02 x 1023

8. Discussion Notes: Standard Temperature and Pressure, STP
One atmosphere of pressure and a temperature of 273 K
Avogadro’s Law: Equal volumes of gases contain equal numbers of gas particles if the temperature and pressure are the same.

9. Definitions: There are exactly 6.02 x 1023 particles in 22.4 L at STP
Mole: 1 mole = 6.02 x 1023 particles
A “particle” can mean: atom, molecule, or formula unit

10. Check for understanding:
2 moles of CO2 = ______________ particles
0.5 mole of He = _____________ particles
3.5 moles of NaCl = _____________ particles

11. Wrap Up: How do chemists keep track of the number of gas particles?
Avogadro’s law states that equal volumes of gases contain the same number of particles if they are at the same temperature and pressure. This holds true for all gases.
Gases are often compared at a standard temperature and pressure of 1 atm and 273 K. This is also referred to as STP.
At STP, any gas will occupy 22.4 L and consist of exactly 6.02 x 1023 particles.

12. Check-in:
One balloon contains 22.4 L of Ar, argon gas, and another balloon contains 22.4 L of Ne, neon gas. Both balloons are at 273 K and 1 atm.
Do the balloons contain the same number of atoms? Why or why not?
Will the balloons have the same mass? Why or why not?

13. Lesson 17: Take a Breath – Ideal Gas Law

14. ChemCatalyst
Describe how you can determine the volume of a breath of air.
Name four factors that might affect the volume you measure.
What do you need to know in order to determine the number of molecules in a breath of air?

15. Key Question
How can you calculate the number of moles of a gas if you know P, V, and T?

16. Objectives: define the ideal gas law
define the universal gas constant, R
complete calculations for finding n, using the ideal gas law

17. Prepare for the Activity:
The equation for the ideal gas law is
PV = nRT
In this equation, R is equivalent to the proportionality constant
R = L · atm/mol · K

18. Discussion Notes:
The ideal gas law allows scientists to relate gas pressure, volume, moles of particles, and temperature.
Note that R is the same for all gases but the value of R does change depending on if the units change.
P must be in Atmospheres
V must be in Liters
n is for “number of moles”
T must be in Kelvin
R is called the Universal Gas Constant

19. Check-In:
You cap a 1.0 L plastic bottle on a mountaintop where the air pressure is 0.50 atm and the temperature is 298 K.
Calculate the number of moles of gas that are in the bottle.