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John E. McMurry • Robert C. Fay C H E M I S T R Y Chapter 9 Gases: Their Properties and Behavior Lecture Notes Alan D. Earhart Southeast Community College • Lincoln, NE Copyright © 2008 Pearson Prentice Hall, Inc. Gases and Gas Pressure Gas mixtures are homogeneous and compressible. © 2012 Pearson Education, Inc. Chapter 9/2 Gases and Gas Pressure Pressure: Force Unit area © 2012 Pearson Education, Inc. Chapter 9/3 Pressure Imbalance in Ear If there is a difference in pressure across the eardrum membrane, the membrane will be pushed out – what we commonly call a “popped eardrum.” Tro, Chemistry: A Molecular Approach 4 Gases and Gas Pressure Barometer Units Pa torr mm Hg atm bar © 2012 Pearson Education, Inc. Chapter 9/5 Gases and Gas Pressure Conversions 1 atm = 760 mm Hg (exact) 1 torr = 1 mm Hg (exact) 1 bar = 1 x 105 Pa (exact) 1 atm = 101 325 Pa © 2012 Pearson Education, Inc. Chapter 9/6 Gases and Gas Pressure The Gas Laws Ideal Gas: A gas whose behavior follows the gas laws exactly. The physical properties of a gas can be defined by four variables: P pressure T temperature (calculation must be in Kelvin) V volume n number of moles Chapter 9/8 The Gas Laws Boyle’s Law 1 V a P (constant n and T) The Gas Laws V a 1 P Boyle’s Law (constant n and T) The Gas Laws V a Boyle’s Law 1 P (constant n and T) PV = k PinitialVinitial = PfinalVfinal © 2012 Pearson Education, Inc. Chapter 9/11 Boyle’s Law and Diving since water is denser than air, for each 10 m you dive below the surface, the pressure on your lungs increases 1 atm at 20 m the total pressure is 3 atm Tro, Chemistry: A Molecular Approach 12 if your tank contained air at 1 atm pressure you would not be able to inhale it into your lungs Examples Calculate the volume of a sample of a gas at 5.75 atm if it occupies 5.14 L at 2.49 atm. (Assume constant temperature) The Gas Laws Charles’ Law Va T (constant n and P) V =k T Vinitial Tinitial Copyright © 2008 Pearson Prentice Hall, Inc. = Vfinal Tfinal Chapter 9/14 The Gas Laws Charles’s Law Va T (constant n and P) The Gas Laws Charles’s Law Va T (constant n and P) V T Vinitial Tinitial =k = © 2012 Pearson Education, Inc. Vfinal Tfinal Chapter 9/16 Examples A sample of argon gas that originally occupied 14.6 L at 25.0oC was heated to 50.0oC at constant pressure. What is its new volume? The Gas Laws Avogadro’s Law Va n (constant T and P) V n Vinitial ninitial Copyright © 2008 Pearson Prentice Hall, Inc. =k = Vfinal nfinal Chapter 9/18 Examples Consider two samples of nitrogen gas (composed N2 molecules). Sample 1 contains 1.5 mol of N2 and has a volume of 36.7 L at 25.0oC and 1 atm. Sample 2 has a volume of 16.0 L at 25.0oC and 1.0 atm. Calculate the number moles of N2 in sample 2 The Ideal Gas Law Summary If the systems is disturbed by one of the four variables: O, T, n then co the following changes Boyle’s Law: Charles’ Law: Avogadro’s Law: PinitialVinitial = PfinalVfinal Vinitial = Vfinal Tinitial Tfinal Vinitial Vfinal = ninitial Copyright © 2008 Pearson Prentice Hall, Inc. nfinal Chapter 9/20 Combine Gas Law is an expression obtained by mathematically combining Boyle’s and Charles’ law P1V1 = P2V2 @ constant n T1 T2 can predict P, V or T when condition is changed Examples Suppose we have a 0.240 mol sample of ammonia gas at 25.0oC with a volume of 3.5 L at a pressure of 1.68 atm. The gas compressed to a volume of 1.35 L at 25.0oC. Use the combined gas law to calculate the final pressure. The Ideal Gas Law Ideal Gas Law: PV = nRT R is the gas constant and is the same for all gases. R = 0.082058 L atm K mol Standard Temperature and Pressure (STP) for Gases T = 0 °C (273.15 K) P = 1 atm Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 9/23 Examples What volume is occupied by 25.7 g of carbon dioxide gas at 25oC and 371 torr? A 0.250 mol sample of argon gas has a volume of 9.00L at a pressure of 875 mmHg. What is the temperature (in oC) of the gas? The Ideal Gas Law Is there a mathematical relationship between P, V, n, and T for an ideal gas? Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 9/25 The Ideal Gas Law What is the volume of 1 mol of gas at STP? (1 mol) V= nRT 0.08206 L atm (273.15 K) K mol = P = 22.41 L (1 atm) Chapter 9/26 © 2012 Pearson Education, Inc. examples A sample of nitrogen gas has a volume of 1.75 L at STP. How many moles of N2 are present? Example Sodium peroxide (Na2O2) is used to remove carbon dioxide from (and add oxygen to) the air supply in spacecrafts. It works by reacting with CO2 in the air to produce sodium carbonate (Na2CO3) and O2. 2 Na2O2(s) + 2 CO2(g) 2 Na2CO3(s) + O2(g)