1. Ch. 11 Molecular Composition of Gases

2. 11-1 Volume-Mass Relationships of Gases
Gay-Lussac’s law of combining volumes of gases-at constant temperature and pressure, the volumes of gaseous reactants and products can be expressed as ratios of small whole numbers
Hydrogen + oxygen ->water vapor
2L L L
2 volumes volume volumes

3. Avogadro’s Law
Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules (doesn’t matter which gas) Fig. 11-1
He discovered that some molecules can have 2 atoms or more (diatomic molecules)

4. Avogadro’s law also indicates gas volume (L) directly proportional to the amount of a gas (n)
V = kn

5. Standard molar volume of a gas
Avogadro’s constant = X 1023 molecules = 1 mole
Standard molar volume of a gas-the volume occupied by one mole of a gas at STP (22.4 L)
Fig mole of each gas occupies 22.4 L but different masses

6. Avogadro’s Law Sample problem 11-1
A chemical reaction produces mol of oxygen gas. What volume in liters is occupied by this gas sample at STP?
mol X L = 1.52 L
1 mol

7. Avogadro’s Law Practice
A sample of hydrogen gas occupies 14.1 L at STP. How many moles of the gas are present?

8. Converting to grams Sample problem 11-2
A chemical reaction produced 98.0 mL of sulfur dioxide gas, SO2, at STP. What was the mass in grams of the gas produced?
.098 L X 1 mol X g SO2 = g
22.4 L mol

9. Converting to grams practice
What is the volume of 77 g of nitrogen dioxide gas at STP?

10. 11-2: The Ideal Gas Law
Mathematical relationship among pressure, volume, temperature, and the number of moles of a gas.
Combination of Boyle’s, Charles’s, Gay-Lussac’s, and Avogadro’s Laws
PV = nRT

11. Ideal gas constant (R), is derived by plugging in all standard values into the equation:
R = PV = nT

12. Ideal gas law sample
What is the pressure in atmospheres exerted by a mol sample of nitrogen gas in a 10 L container at 298 K?
Answer = 1.22 atm

13. More ideal gas law practice
What is the volume, in liters, of mol of oxygen gas at 20°C and atm pressure?
Answer = 6.17 L

14. Sample problem 11-5
What mass of chlorine gas, Cl2, in grams, is contained in a 10 L tank at 27°C and 3.5 atm of pressure?
Answer = 101 g

15. Finding molar mass or density
PV = mRT M
M = mRT M = DRT
PV P
D = MP RT

16. Sample Problem
At 28°C and atm, 1.00 L of gas has a mass of 5.16 g. What is the molar mass of this gas?

17. 11-3 Stoichiometry of Gases
Coefficients can be used as volume ratios:
2CO + O2 -> 2CO2
2 volumes CO
1 volume O2

18. Sample Problem 11-7 volume-volume

19. Sample problem 11-8 volume-mass

20. Sample problem 11-9

21. 11-4 Effusion and Diffusion
Graham’s Law of Effusion-rates of diffusion and effusion depend on the relative velocities of gas molecules
Rates of effusion of gases at the same temperature and pressure are inversely proportional to the square roots of their molar masses.

22. Graham’s Law formula Rate of effusion A = √MB Rate of effusion B √MA
Molar masses can also be replaced by densities of the gases:
Rate of effusion A = √densityB
Rate of effusion B √denistyA

23. Graham’s Law Problem Sample problem 11-10
Compare the rates of effusion of hydrogen and oxygen at the same temperature and pressure. (smaller molar mass gas will diffuse faster-how much faster?)
Smaller molar mass goes on bottom

24. Diffusion Quicklab pg. 353