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Chapter 11

Chapter 11. Molecular Composition of Gases. N 2. +. 3 H 2. →. 2 NH 3. 1 volume. →. 2 volumes. 3 volumes. +. 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.

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Chapter 11

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  1. Chapter 11 Molecular Composition of Gases

  2. N2 + 3 H2 → 2 NH3 1 volume → 2 volumes 3 volumes + 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.

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

  6. Mole-Mass-Volume Relationships

  7. 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.

  8. Find the mass in grams of 2.80 L of carbon dioxide? • (2.8 L CO2)(1 mol CO2/22.4 L CO2)(44 g CO2 /1 mol CO2 ) = 5.5 g CO2

  9. Density of Gases grams liters

  10. Density of Gases depends on T and P

  11. The molar mass of SO2 is 64.07 g/mol. Determine the density of SO2 at STP. 1 mole of any gas occupies 22.4 L at STP

  12. nT V a P Ideal Gas Equation

  13. atmospheres nT V a P

  14. liters nT V a P

  15. moles nT V a P

  16. Kelvin nT V a P

  17. nT Ideal Gas Constant V a P

  18. A balloon filled with 5.00 moles of helium gas is at a temperature of 25oC. The atmospheric pressure is 0.987 atm. What is the balloon’s volume? Step 1. Organize the given information. Convert temperature to kelvins. K = oC + 273 K = 25oC + 273 = 298K

  19. A balloon filled with 5.00 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.

  20. Determination of Molecular Weights Using the Ideal Gas Equation

  21. Calculate the molar mass of an unknown gas, if 0.020 g occupies 250 mL at a temperature of 305 K and a pressure of 0.045 atm. V = 250 mL = 0.250 L g = 0.020 g P = 0.045 atm T = 305 K

  22. Determination of Density Using the Ideal Gas Equation • Density = mass/volume D = MP/ RT

  23. The density of a gas was measured at 1.50 atm and 27 ºC and found to be 1.95 g/L. Calculate the molar mass of the gas. • (1.95g/L)(0.08206 L atm/K mol)(300K) 1.50 atm M = dRT P = 32.0 g/mol

  24. 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.

  25. Gas Stoichiometry Primary conversions involved in stoichiometry.

  26. What volume of oxygen (at STP) can be formed from 0.500 mol of potassium chlorate? • Step 1 Write the balanced equation 2 KClO3 2 KCl + 3 O2 • Step 2 The starting amount is 0.500 mol KClO3. The conversion is moles KClO3 moles O2 liters O2

  27. What volume of oxygen (at STP) can be formed from 0.500 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

  28. What volume of oxygen (at STP) can be formed from 0.500 mol of potassium chlorate? The problem can also be solved in one continuous calculation. 2 KClO3 2KCl + 3 O2

  29. What volume of hydrogen, collected at 30.oC and 0.921 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

  30. What volume of hydrogen, collected at 30.oC and 0.921 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 + 273 = 303 K

  31. 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

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

  33. 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)

  34. Mass- volume problem

  35. 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.

  36. 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

  37. Calculate the ratio of the effusion rates of H2 & UF6, a gas used in the enrichment process to produce fuel for nuclear reactors. • Rate of effusion of H2 • Rate of effusion of UF • Square root of molar mass of UF6 • Square root of molar mass of H2 Square root of 352.02/2.016 = 13.2 13.2

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