Chapter 6 Gases PV=nRT is SUPER- FANTASTIC!!! 1 I. Properties of Gases A) Physical Properties B) Pressure II. The Ideal Gas Law A) PV=nRT Calculations B) Molar Mass and Density Calculations C) Stoichiometry D) Partial Pressures and Mole Fractions III. Kinetic Molecular Theory A) Effusion and Diffusion B) Real Gases vs. Ideal Gases http://www.nasa.gov/sites/default/files/1-bluemarble_west.jpg, https://upload.wikimedia.org/wikipedia/commons/thumb/a/a3/atmosphere_composition_diagramen.svg/1024px-atmosphere_composition_diagram-en.svg.png
Stoichiometry Problems Thus Far 2 1. S 8 (s) + 8Mg(s) 8MgS(s) 2. HCl(aq) + AgNO 3 (aq) AgCl(s) + HNO 3 (aq) 3. N 2 (g) + 3H 2 (g) 2NH 3 (g) How do we quantify amounts of the gases reacted and produced?
I. Properties of Gases A. Physical Properties 3 1. gases expand to fill the container holding them - gas particles are much smaller than the container holding them V gas = Volume of free space the gas can occupy ~ Volume of the container that holds the gas 2. compressible 3. speed of gas particles directly related to temperature 4. form homogeneous mixtures - uniform composition throughout - normal conditions gas particles move very fast and are relatively far away from each other https://static.squarespace.com/static/4ff36a2b84aecc34311d0e6c/523b0fcce4b099ee151514e7/523b0fd5e4b099ee151523bf/1350328192307/1000w/red%20bull %20Stratos%203.jpg
B. Pressure (at the molecular level) 4 Pressure Meter Measures frequency of collisions and force of collisions What happens to the pressure? http://img.bleacherreport.net/img/slides/photos/003/680/493/hi-res-117182f11c7a20c6f3353a5b52b62144_crop_north.jpg?w=630&h=420&q=75
Units of pressure 5 1 atm (atmosphere) = pressure at the surface of the Earth
II. The Ideal Gas Law 6 P nt V Units R = 0.082057 L atm / mol K P = pressure (atm) V = volume (L) n = # of moles T = temperature (K) What is an ideal gas? Assumptions 1. Molecules/gas particles are assumed to occupy an infinitesimally small volume relative to the size of the container 2. Molecules/gas particles are assumed to be independant of each other
A. Simple PV=nRT Calculations Example Problem Jacques Charles was the first to create a hot air balloon. However, instead of using air he used H 2 gas. His first balloon was filled at sea-level with 262 g of H 2 at a temperature of 23.0 ºC, atomospheric pressure was 750.0 torr. What was the volume of the balloon? 7
8 Example Problem Helium filled balloons are used to carry scientific instruments high into the atmosphere. Suppose a balloon is launched when the temperature is 22.5ºC and the barometric pressure is 754 mmhg. When it is filled, the balloon s volume is 4.19x10 3 L. What will the volume be at a height of 20 km, where the pressure is 76.0 mmhg and the temperature is 33.0ºC (assume no helium leaks from the balloon)? Initial Conditions Final Conditions P 1 = 754 mmhg P 2 = 76.0 mmhg V 1 = 4.19x10 3 L V 2 = n 1 = llllllllllllll n 2 = n 1 = constant T 1 = 22.5ºC (295.6 5 K) T 2 = -33.0ºC (240.1 5K) http://www.nasa.gov/sites/default/files/thumbnails/image/launch.jpeg
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Standard Temperature and Pressure 10 https://upload.wikimedia.org/wikipedia/en/5/57/stp_logo.png
Molar Mass and Density Calculations 11 Goal Use PV = nrt to find density and/or molar mass of a gas
Using PV=nRT to calculate 12 Molar Mass Density
Example Problem A mixture of unknown gas and oxygen was used as an anesthetic on the t.v. program LOST. The unknown gas contains 85.7% C and 14.3 % H by mass. At 50.0 C and 0.984 atm. of pressure, 1.56 g of the gas has a volume of 1.00 L. What is the molecular formula of the gas? 13 https://images2.alphacoders.com/750/750.jpg
Example Problem Calculate the density of CO 2 (g) at STP. 14
C. Gases in Stoichiometry Problems Example Problem Ammonium sulfate, an important fertilizer, can be prepared by the reaction of ammonia with sulfuric acid: NH 3 (g) + H 2 SO 4 (aq) (NH 4 ) 2 SO 4 (aq) What mass of (NH 4 ) 2 SO 4 can be produced from the reaction of 5400. L of NH 3 (g) at 42.0ºC and 8.6 atm with 1000. L of 0.8000 N H 2 SO 4 (aq). NH 3 (g) + H 2 SO 4 (aq) (NH 4 ) 2 SO 4 (aq) 15
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17 D. Partial Pressures and Mole Fractions All gases have the same volume and same T Individual Gas (O 2 )
Example Problem A mixture containing He(g) at a pressure of 1.88 atm, Ne(g) at a pressure of 1.10 atm, and Ar(g) at a pressure of 0.360 atm is confined in a 7.00 L vessel at 25 C. What is the mole fraction of He in the mixture? How many moles of gases are in the mixture? 18
Example Problem A study of the effects of certain gases on plant growth requires a synthetic atmosphere composed of 1.50 mol percent CO 2, 18.0 mol percent O 2, and 80.5 mol percent Ar. (a) Calculate the partial pressure of O 2 in the mixture if the total pressure of the synthetic atmosphere is to be 745 torr. (b) If this atmosphere is to be held in a 120 L space at 295 K, how many moles of O 2 are needed? 19
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21 Example Problem A sample of KClO 3 partially decomposes according to the equation below. The O 2 (g) that is produced is collected in a container over water. The volume of the collected gas is 0.250 L at a temperature of 25.0ºC. If the total pressure in the container is 765.0 torr after the KClO 3 decomposes, (a) how many moles of O 2 (g) are collected, and (b) how many moles of KClO 3 were decomposed? KClO 3 (s) KCl(s) + O 2 (g)
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