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Chapter 8 Gases

Chapter 8 Gases. 8.1 Gases and Kinetic Theory 8.2 Gas Pressure 8.8 Ideal Gas Law

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Chapter 8 Gases

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  1. Chapter 8 Gases 8.1 Gases and Kinetic Theory 8.2 Gas Pressure 8.8 Ideal Gas Law * You do not need to know Boyle’s (8.3), Charles’ (8.4), Gay-Lussac’s (8.5), Avogadro’s (8.7) or the Combined gas (8.6) laws. They are all contained within the ideal gas law. We will not cover Dalton’s law (8.9).

  2. 8.1 Kinetic Theory of Gases Particles of a gas • Move rapidly in straight lines. • Have kinetic energy that increases with an increase in temperature. • Are very far apart. • Have essentially no attractive (or repulsive) forces. • Have very small volumes compared to the volume of the container they occupy.

  3. Properties of Gases • Gases are described in terms of four properties: pressure (P), volume (V), temperature (T), and amount (n).

  4. 8.2 Barometer • A barometer measures the pressure exerted by the gases in the atmosphere. • The atmospheric pressure is measured as the height in mm of the mercury column.

  5. Learning Check A. The downward pressure of the Hg in a barometer is _____ than/as the weight of the atmosphere. 1) greater 2) less 3) the same B. A water barometer is 13.6 times taller than a Hg barometer (DHg = 13.6 g/mL) because 1) H2O is less dense 2) H2O is heavier 3) air is more dense than H2O

  6. Solution A.The downward pressure of the Hg in a barometer is 3) the same as the weight of the atmosphere. B. A water barometer is 13.6 times taller than a Hg barometer (DHg = 13.6 g/mL) because 1) H2O is less dense

  7. Pressure • A gas exerts pressure, which is defined as a force acting on a specific area. Pressure (P) = Force Area • One atmosphere (1 atm) is 760 mm Hg. • 1 mm Hg = 1 torr1.00 atm = 760 mm Hg = 760 torr

  8. Units of Pressure • In science, pressure is stated in atmospheres (atm), millimeters of mercury (mm Hg), and Pascals (Pa).

  9. Learning Check A. What is 475 mm Hg expressed in atm?1) 475 atm2) 0.638 atm3) 3.61 x 105 atm B. The pressure in a tire is 2.00 atm. What is this pressure in mm Hg?1) 2.00 mm Hg2) 1520 mm Hg3) 22,300 mm Hg

  10. Solution A. What is 475 mm Hg expressed in atm? 2) 0.638 atm 485 mm Hg x 1 atm = 0.638 atm 760 mm Hg B. The pressure of a tire is measured as 2.00 atm. What is this pressure in mm Hg? 2) 1520 mm Hg 2.00 atm x 760 mm Hg = 1520 mm Hg 1 atm

  11. 8.8 Ideal Gas Law • The relationship between the four properties (P, V, n, and T) of gases can be written equal to a constant R. PV = R nT • Rearranging this expression gives the expression called the ideal gas law. PV = nRT

  12. STP • The volumes of gases can be compared when they have the same conditions of temperature and pressure (STP). Standard temperature (T) 273 K (0°C ) Standard pressure (P) 1 atm (760 mm Hg)

  13. Molar Volume • At STP, 1 mole of a gas occupies a volume of 22.4 L. • The volume of one mole of a gas is called the molar volume.

  14. Molar Volume as a Conversion Factor • The molar volume at STP can be used to form conversion factors.22.4 L and 1 mole 1 mole 22.4 L

  15. Learning Check A. What is the volume at STP of 4.00 g of CH4? 1) 5.60 L 2) 11.2 L 3) 44.8 L B. How many grams of He are present in 8.00 L of gas at STP? 1) 25.6 g 2) 0.357 g 3) 1.43 g

  16. Solution A. 1) 5.60 L 4.00 g CH4 x 1 mole CH4 x 22.4 L (STP) = 5.60 L 16.0 g CH4 1 mole CH4 B. 3) 1.43 g 8.00 L x 1 mole He x 4.00 g He = 1.43 g He 22.4 L 1 mole He

  17. Universal Gas Constant, R • The universal gas constant, R, can be calculated using the molar volume of a gas at STP. • At STP (273 K and 1.00 atm), 1 mole of a gas occupies 22.4 L. P V R = PV = (1.00 atm)(22.4 L) nT (1 mole) (273K) n T = 0.0821 L atm mole K • Note there are four units associated with R.

  18. Learning Check Another value for the universal gas constant is obtained using mm Hg for the STP pressure. What is the value of R when a pressure of 760 mm Hg is placed in the R value expression?

  19. Solution What is the value of R when the STP value for P is 760 mmHg? R = PV = (760 mm Hg) (22.4 L) nT (1 mole) (273K) = 62.4 L mm Hg mole K

  20. Learning Check Dinitrogen oxide (N2O), laughing gas, is used by dentists as an anesthetic. If a 20.0 L tank of laughing gas contains 2.8 moles N2O at 23°C, what is the pressure (mm Hg) in the tank?

  21. Solution 1. Adjust the units of the given properties to match the units of R. V = 20.0 L, T = 296 K, n = 2.8 moles, P = ? 2. Rearrange the ideal gas law for P. P = nRT V P = (2.8 moles)(62.4 L mm Hg)(296 K) (20.0 L) (mole K) = 2.6 x 103 mm Hg

  22. Learning Check A cylinder contains 5.0 L of O2 at 20.0°C and 0.85 atm. How many grams of oxygen are in the cylinder?

  23. Solution 1. Determine the given properties. P = 0.85 atm, V = 5.0 L, T = 293 K, n (or g =?) 2. Rearrange the ideal gas law for n (moles). n = PV RT = (0.85 atm)(5.0 L)(mole K) = 0.18 mole O2 (0.0821atm L)(293 K) 3. Convert moles to grams using molar mass. = 0. 18 mole O2 x 32.0 g O2 = 5.8 g O2 1 mole O2

  24. Molar Mass of a Gas What is the molar mass of a gas if 0.250 g of the gas occupy 215 mL at 0.813 atm and 30.0°C? 1. Solve for the moles (n) of gas. n = PV = (0.813 atm) (0.215 L) RT (0.0821 L atm/mole K)(303K) = 0.00703 mole 2. Set up the molar mass relationship. Molar mass = g = 0.250 g = 35.6 g/mole mole 0.00703 mole

  25. Gases in Equations • The amounts of gases reacted or produced in a chemical reaction can be calculated using the ideal gas law and mole factors. Problem: What volume (L) of Cl2 gas at 1.2 atm and 27°C is needed to completely react with 1.5 g of aluminum? 2Al(s) + 3Cl2 (g) 2AlCl3(s)

  26. Gases in Equations (continued) 2Al(s) + 3Cl2 (g) 2AlCl3(s) 1.5 g ? L 1.2 atm, 300K 1. Calculate the moles of Cl2 needed. 1.5 g Al x 1 mole Al x 3 moles Cl2 = 0.083 mole Cl2 27.0 g Al 2 moles Al 2. Place the moles Cl2 in the ideal gas equation. V = nRT = (0.083 mole Cl2)(0.0821 Latm/moleK)(300K) P 1.2 atm = 1.7 L Cl2

  27. Learning Check What volume (L) of O2 at24°C and 0.950 atm are needed to react with 28.0 g NH3? 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

  28. Solution 1. Calculate the moles of O2 needed. 28.0 g NH3 x 1 mole NH3 x 5 mole O2 17.0 g NH3 4 mole NH3 = 2.06 mole O2 2. Place the moles O2 in the ideal gas equation. V = nRT = (2.06 moles)(0.0821 L atm/moleK)(297K)P 0.950 atm = 52.9 L O2

  29. Solution V1 = V2 T1 T2 Cross multiply to give V1T2 = V2T1 Isolate T2 by dividing through by V1 V1T2 = V2T1 T2 = V2T1 V1 V1 V1

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