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Ch. 11 Molecular Composition of Gases

Ch. 11 Molecular Composition of Gases. 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

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Ch. 11 Molecular Composition of Gases

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  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 1L 2L 2 volumes 1 volume 2 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 = 6.022 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. 11-3 1 mole of each gas occupies 22.4 L but different masses

  6. Avogadro’s Law Sample problem 11-1 • A chemical reaction produces 0.0680 mol of oxygen gas. What volume in liters is occupied by this gas sample at STP? • 0.0680 mol X 22.4 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 64.07 g SO2 = 0.280 g 22.4 L 1 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 = 0.0821 nT

  12. Ideal gas law sample • What is the pressure in atmospheres exerted by a 0.500 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 0.250 mol of oxygen gas at 20°C and 0.974 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 0.974 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

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