CHAPTER 11 GASES Problems 1 6 8 11 12 14 22 23 32 skip problems with Pa 33 88 99 11 1 Extra Long Straws Example Consider what you know about atmospheric pressure and gas pressure to explain why it s easier to drink soda with a straw than to drink a milkshake with a straw 11 2 KINETIC MOLECULAR THEORY A MODEL FOR GASES 1 Particles in a gas are so tiny they take up no volume 2 Gases consist of particles in constant straight line motion 3 Particles are neither attracted nor repelled by one another Particles do not stick together so the total number of particles remains constant 4 Particles transfer energy completely during collisions and no energy is lost to friction 5 As temperature increases the kinetic energy and thus the speed of the particles increases CHEM 121 Chapter 11 page 1 of 9 Properties of Gases 1 Gases have an indefinite shape They take the shape of their container 2 Gases can expand or compress If the volume is increased the gas expands to fill the larger volume If the volume is decreased the gas particles move closer together 3 Gases have low densities About 1000 times less dense than liquids and solids Why density of gases reported in g L 4 Gases will mix in their containers to form uniform mixtures due to their kinetic energy 5 A gas confined to a container will exert pressure on the container walls since the particles are constantly in motion 11 4 11 5 and 11 6 BOYLE S LAW CHARLES LAW and AVOGADRO S LAW gas pressure force per unit area exerted by gas molecules colliding against the inside walls of their container a measure of how often gas particles hit the container walls Variables Affecting Gas Pressure What happens to the pressure of a gas given the following changes Circle one for each below a If the container Volume pressure Thus pressure and volume are related b Temperature pressure not directly stays same inversely stays same Thus pressure and temperature are related not directly c of gas molecules pressure inversely stays same Thus pressure and of molecules are related not CHEM 121 Chapter 11 directly inversely page 2 of 9 11 6 THE COMBINED GAS LAW PRESSURE VOLUME AND TEMPERATURE standard atmospheric pressure a column of mercury measuring 760 mm Hg 1 atm 760 torr 760 mm Hg 14 7 psi approx Example In Denver the atmospheric pressure was measured to be 632 mmHg What is the pressure in torr and atm The Combined Gas Law allows us to calculate pressure P volume V or temperature T for a gas when there is a change in one or more variables Guidelines for solving gas law problems 1 List all of the measurements given and label each as P1 V1 T1 etc P V 2 Starting with the Combined Gas Law 1 1 T1 P2 V2 cancel variables that remain constant T2 3 Isolate the unknown variable 4 Convert all temperatures to Kelvins and all pressures and or volumes to the same units 5 Solve for the unknown making sure your final answer has the correct units and the correct number of sig figs 6 Check if your final answer makes sense Exercises If a value P V or T is not given that value remains constant 1 If 25 0 mL of hydrogen gas are heated from 225K to 675K calculate the new volume CHEM 121 Chapter 11 page 3 of 9 2 A sample of CO2 gas at 2 50 atm is cooled from 75 C to 25 C Calculate the new pressure 3 A diver at a depth of 20 m inhales 0 50 L of an air mixture at a pressure of 3 00 atm If the diver holds his breath and quickly swims to the surface where the pressure is 1 00 atm calculate the new volume of air in his lungs Explain why doing this would be dangerous 4 A 5 00 L sample of neon gas at 25 C and 0 975 atm is heated to 325K If the new volume of the gas is 10 0 L calculate the new pressure for the gas 11 8 THE IDEAL GAS LAW PV nRT where P pressure in atm V volume in L n of moles of gas L atm T temperature in K and the ideal gas constant R 0 0821 mol K Ex 1 How many moles of NO2 gas occupy a volume of 5 00 L at 45 C and 725 mmHg CHEM 121 Chapter 11 page 4 of 9 Ex 2 What mass of CO gas occupies a volume of 75 0 L at 35 C and 2 50 atm Standard Temperature and Pressure abbreviated STP T 0 00 C and P 1 00 atm Ex 3 Calculate the volume for 1 00 mole of gas at STP 11 10 Gases in Chemical Reactions Molar Volume at Standard Temperature and Pressure Calculations for Gases at STP Know that the molar volume of an ideal gas at STP 0 C and 1 atm is always 22 4 L mol Ex 1 Calculate the density in g L of carbon dioxide gas at STP Hint Knowing the formula of a gas means you can determine what property Ex 2a Calculate the molar mass of a gas with a density of 1 25 g L at STP b If the unknown gas described in part a is an element identify the gas CHEM 121 Chapter 11 page 5 of 9 11 9 MIXTURES OF GASES WHY DEEP SEA DIVERS BREATHE A MIXTURE OF HELIUM AND OXYGEN Dalton s Law of Partial Pressure partial pressure pressures of individual gas components in a mixture Dalton s Law of Partial Pressure total pressure of a mixture of gases is the sum of the partial pressures of the gases present PTotal P1 P2 P3 Ex 1 Air is a mixture of nitrogen oxygen and trace gases If the atmospheric pressure on a given day is 0 998 atm and the partial pressure of N2 and O2 are 0 785 atm and 0 177 atm respectively calculate the partial pressure of the trace gases Partial Pressure and Too Little or Too Much Oxygen At sea level total atmospheric pressure is about 1 00 atm Air is about 78 nitrogen N2 and 21 oxygen O2 by volume our lungs have evolved to breathe O2 at a partial pressure of about 0 21 atm CHEM 121 Chapter 11 page 6 of 9 Too Little Oxygen At higher altitudes where atmospheric pressure is much lower the partial pressure of O2 is also lower e g on the top of Mt Everest total atmospheric pressure is only about 0 311 atm so the partial pressure of O2 is only about 0 065 atm hypoxia a condition where the body does not have a sufficient oxygen supply which can lead to dizziness headache and shortness of breath Too Much Oxygen Due to the weight of water above him a scuba diver experiences an additional 1 atm of pressure for every 10 m he descends The pressure regulators used by …