1. Molecular Composition of Gases
Chapter 11
Molecular Composition of Gases

2. Gay Lussac’s Law of Combining Volumes
When measured at the same temperature and pressure, the ratio of the volumes of reacting gases are small whole numbers. N2
1 volume +
3 H2
3 volumes →
2 NH3
2 volumes
1. Students write balanced equation for reaction. Note balancing coefficients are same as volume in Liters.

3. Gay Lussac’s Law of Combining Volumes
When measured at the same temperature and pressure, the ratio of the volumes of reacting gases are small whole numbers.

4. Avogadro’s Law
Equal volumes of different gases at the same temperature and pressure contain the same number of molecules.

5. hydrogen + chlorine → hydrogen chloride
H2 + Cl2 → 2 HCl
1 volume 1 volume volumes
1 molecule 1 molecule molecules
1 mol mol mol
hydrogen + chlorine → hydrogen chloride
Each molecule of hydrogen and each molecule of chlorine contains 2 atoms.

6. Mole-Mass-Volume Relationships

7. 22.4 L at STP is known as the molar volume of any gas.
Mole-Mass-Volume Relationships
Volume of one mole of any gas at STP = 22.4 L.
22.4 L at STP is known as the molar volume of any gas.

9. How many moles of ammonia gas occupy 56 Liters at STP?

10. Find the volume of 89.5 grams of carbon dioxide at STP.

11. Find the volume of 4.79 moles of oxygen at STP

12. Density of Gases
grams
liters

13. Density of Gases
depends on T and P

14. 1 mole of any gas occupies 22.4 L at STP
The molar mass of SO2 is g/mol. Determine the density of SO2 at STP.
1 mole of any gas occupies 22.4 L at STP

15. Find the density of carbon dioxide at STP.

16. Find the molar mass of a gas that has a density of 1.425 g/L at STP.

17. nT V a P Ideal Gas Equation
Mathematical relationship of pressue, volume, temperature and number of moles of a gas

18. atmospheres nT
V a P
Number of molecules or moles affects other three quatities. Increase #, increase collision rate, increase pressure.

19. liters nT
V a P

20. moles nT
V a P

21. Kelvin nT
V a P

22. nT V a P Ideal Gas Constant
R= ideal gas constant. has the same value for all gases whose behavior approaches that of an ideal gas. Solve for R at STP. P= 1 atm, V= 22.4, n= 1 mol, T= 273 K

23. A balloon filled with moles of helium gas is at a temperature of 25oC. The atmospheric pressure is atm. What is the balloon’s volume?
Step 1. Organize the given information.
Convert temperature to kelvins.
K = oC + 273
K = 25oC = 298K

24. Step 3. Substitute the given data into the equation and calculate.
A balloon filled with moles of helium gas is at a temperature of 25oC. The atmospheric pressure is .987 atm. What is the balloon’s volume?
Step 2. Write and solve the ideal gas equation for the unknown.
Step 3. Substitute the given data into the equation and calculate.

25. How many moles of oxygen occupy 250
How many moles of oxygen occupy mL at 25 degrees Celsius and 720 torr?

26. What volume does 0. 500 grams of hydrogen occupy at
What volume does grams of hydrogen occupy at .835 atm and 300 Kelvin?

27. Gases are assumed to behave as ideal gases.
Gas Stoichiometry
deals with the quantitative relationships among reactants and products in a chemical reaction.
All calculations are done at STP.
Gases are assumed to behave as ideal gases.
A gas not at STP is converted to STP.

28. Gas Stoichiometry Primary conversions involved in stoichiometry.
Coefficients for gaseous reactants or products indicate molar amounts, mole ratios and volume ratios.
Primary conversions involved in stoichiometry.

29. What volume of oxygen (at STP) can be formed from 0
What volume of oxygen (at STP) can be formed from mol of potassium chlorate?
Step 1 Write the balanced equation
2 KClO3  2 KCl + 3 O2
Step 2 The starting amount is mol KClO3. The conversion is
moles KClO3  moles O2  liters O2

30. What volume of oxygen (at STP) can be formed from 0
What volume of oxygen (at STP) can be formed from mol of potassium chlorate?
2 KClO3  2KCl + 3 O2
Step 3. Calculate the moles of O2, using the mole-ratio method.
Step 4. Convert moles of O2 to liters of O2

31. The problem can also be solved in one continuous calculation.
What volume of oxygen (at STP) can be formed from mol of potassium chlorate?
The problem can also be solved in one continuous calculation.
2 KClO3  2KCl + 3 O2

32. 2 Al(s) + 6 HCl(aq)  2AlCl3(aq) + 3 H2(g)
What volume of hydrogen, collected at 30.oC and atm, will be formed by reacting 50.0 g of aluminum with hydrochloric acid?
2 Al(s) + 6 HCl(aq)  2AlCl3(aq) + 3 H2(g)
Step 1 Calculate moles of H2.
grams Al  moles Al  moles H2

33. Step 2 Calculate liters of H2.
What volume of hydrogen, collected at 30.oC and atm, will be formed by reacting 50.0 g of aluminum with hydrochloric acid?
2 Al(s) + 6 HCl(aq)  2AlCl3(aq) + 3 H2(g)
Step 2 Calculate liters of H2.
Convert oC to K: 30.oC = 303 K

34. Solve the ideal gas equation for V
What volume of hydrogen, collected at 30.oC and 700. torr, will be formed by reacting 50.0 g of aluminum with hydrochloric acid?
Solve the ideal gas equation for V
PV = nRT

35. For reacting gases at constant temperature and pressure: Volume-volume relationships are the same as mole-mole relationships.
H2(g) Cl2(g)  HCl(g)
1 mol H2
1 mol Cl2
2 mol HCl
22.4 L
STP
22.4 L
STP
2 x L
STP
1 volume
1 volume
2 volumes

36. What volume of nitrogen will react with 600
What volume of nitrogen will react with 600. mL of hydrogen to form ammonia? What volume of ammonia will be formed?
N2(g) + 3H2(g)  2NH3(g)

37. Mass- volume problem
How many liter (at STP) of hydrogen are produced when 1.43 g zinc reacts with hydrochloric acid?

38. Volume-Mass Problem
How many grams of sodium react with 25.3 liters of chlorine (at STP) to produce sodium chloride?

39. How many liters of hydrogen at 1
How many liters of hydrogen at 1.25 atm and 75 degrees Celsius are produced when 1.68 g of magnesium reacts with hydrochloric acid? (This is not at STP!!)
Find mole of hydrogen produced
Use PV=nRT to find liters of hydrogen

40. Graham’s Law of Effusion
Effusion – the process whereby the molecules of a gas confined in a container randomly pass through a tiny opening in the container.
Rate (like diffusion) depends on the relative velocities of gas molecules.
Lighter molecules move faster than heavier molecules at the same temperature.

41. Graham’s Law of Effusion
The rates of effusion of gases at the same temperature and pressure are inversely proportional to the square roots of their molar masses.
Rate of effusion of A = MB
Rate of effusion of B MA
M = molar masses, density of a gas varies directly with molar mass, can replace molar mass with density

42. Find the molar mass of the gas which effuses 3
Find the molar mass of the gas which effuses 3.98 times faster than oxygen.