1. Molecular Composition of Gases
Chapter 12
Molecular Composition of Gases

2. Molar Volume of a Gas
One mole of a gas has the same volume at STP as any other gas
22.4 L / mole at STP

3. Practice Problem
What volume would mol of oxygen gas occupy at STP?
What about mol of nitrogen gas at STP?

4. Ideal Gas Law PV = nRT n = Number of Moles R = Ideal Gas Constant
L . atm/ mol . K
V = Volume (must be in liters)
P = Pressure (must be in atmospheres)
T = Temperature (must be in Kelvin)

5. Practice Problem
What is the P (in atm) exerted by a mol sample of nitrogen gas in a 10.0 L container at 298 K?
PV = nRT rearranges to P= nRT/ V
(0.500 mol)( L .atm/mol .K)(298K)
10.0 L
Pressure = atm

6. Gas Stoichiometry Uses volume-volume calculations (e.g. L  L)
Liters can be used just like mole to mole ratios in a factor label problem

7. C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g)
Practice Problem
How many L of oxygen are required for the complete combustion of L of propane?
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g)
Ratio: 5 L O2 makes 1 L C3H8
0.250 L C3H8 x 5 L O2
1 L C3H8
= 1.25 L O2

8. CaCO3(s)  CaO(s) + CO2(g)
Practice Problem
How many grams of calcium carbonate must be decomposed to produce 2.00 L of CO2 at STP?
CaCO3(s)  CaO(s) + CO2(g)
2.00 L CO2 x 1 mol CO2 x 1 mol CaCO3
22.4 L CO mol CO2
x g CaCO3
1 mol CaCO3
8.94 g CaCO3

9. Graham’s Law
Diffusion
Effusion

10. Graham’s Law of Effusion or Diffusion
The rate of effusion or diffusion is inversely proportional to the square roots of their molar masses
Rate A = MB
Rate B MA

11. Practice Problem
Compare the rate of effusion of hydrogen and nitrogen gas
rate H2 = = = 3.7
rate N
Hydrogen effuses 3.7 times faster than nitrogen