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The Chemistry of Gases

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A Gas -Uniformly fills any container. -Mixes completely with any other gas -Exerts pressure on its surroundings.

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Pressure -is equal to force/unit area -SI units = Newton/meter 2 = 1 Pascal (Pa) -1 standard atmosphere = 101,325 Pa -1 standard atmosphere = 1 atm = 760 mm Hg = 760 torr

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Effect of Atmospheric Pressure

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Standard Pressure Units 1 atmosphere = 760 mm Hg = 760 torr x 10 5 Pa = kPa 14.7 psi (pounds per square inch)

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Pressure Conversions The pressure of a gas is measured as 49 torr. Represent this as atmospheres and as Pascals. 49 torr| 1 atm = 6.4 x atm | 760 torr 49 torr | x 10 5 Pa = 6.5 x 10 3 Pa | 760 torr

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Boyles Law * Pressure Volume = Constant (T = constant) P 1 V 1 = P 2 V 2 (T = constant) V 1/P (T = constant) ( * Holds precisely only at very low pressures.)

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A gas that strictly obeys Boyles Law is called an ideal gas.

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1.53 L of sulfur dioxide gas at 5600 Pa is changed to Pa at constant temperature. What is its new volume? P 1 V 1 =P 2 V x 1.53 = x V 2 V 2 = 0.57 L 1.0 L of hydrogen gas at 345 torr is connected to an empty 2.5 L flask and allowed to combine. What is its new pressure? 345 x 1.0 = P 2 x 3.5 P 2 = 98.6 torr

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Charless Law The volume of a gas is directly proportional to temperature, and extrapolates to zero at zero Kelvin. V = bT (P = constant) b = a proportionality constant

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Balloon with Liquid N 2

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Charless Law

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Always Change to KELVIN! K = C + 273

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A sample of gas at 15 o C and 1 atm has a volume of 2.58 L. What is its new volume at 38 o C and 1 atm pressure? Pressure remains constant at 1 atm T 1 = = 288 K T 2 = = 311 K V 1 = V 2 or V 1 T 2 = V 2 T 1 T 1 T x 311 = V 2 x 288 V 2 = 2.79 L

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Gay-Lussacs Law Pressure is directly proportional to the Kelvin Temperature P = k T or P 1 = P 2 T 1 T 2

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Standard Temperature and Pressure STP P = 1 atmosphere T = C The molar volume of an ideal gas is liters at STP

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Combined Gas Law When all three change use: P 1 V 1 = P 2 V 2 T 1 T 2 or P 1 V 1 T 2 = P 2 V 2 T 1

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A cylinder contains 255 mL of CO 2 at 27 o C and 812 torr. What volume would this occupy at STP? P 1 = 812 torr T 1 = 27 o C + 273= 300 K V 1 = 255 mL P 2 = 1 atm= 760 torr T 2 = 0 o C = 273 K P 1 V 1 T 2 =P 2 V 2 T 1 (812)(255)(273)=V 2 (760)(300) V 2 = mL

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Avogadros Law For a gas at constant temperature and pressure, the volume is directly proportional to the number of moles of gas (at low pressures). V = an a = proportionality constant V = volume of the gas n = number of moles of gas

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Ideal Gas Law -An equation of state for a gas. -state is the condition of the gas at a given time. PV = nRT

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Ideal Gas Law PV = nRT R = proportionality constant = L atm mol P = pressure in atm V = volume in liters n = moles T = temperature in Kelvins Holds closely at P < 1 atm

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Ideal Gas Laws Can be used to calculate molar masses by gas density density= mass = molar mass volume molar volume At STP, molar volume is 22.4 L

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Suppose you have a sample of ammonia gas with a volume of 856 mL at a pressure of 932 torr and 28 o C. What is the mass of the ammonia present? P= 932 torr / 760 torr = 1.23 atm V = L T = = 301 K n = PV = (1.23)(.856) = moles RT(.0821)(301) mass = moles x MM= x 17= 0.724g

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Daltons Law of Partial Pressures For a mixture of gases in a container, P Total = P 1 + P 2 + P Used primarily when collecting gases over water.

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Mixtures of helium and oxygen are used for deep scuba diving. If 46 L oxygen and 12 L He at 1.0 atm are put into a 5.0 L tank at 25 o C, calculate the partial pressures of each gas and the total pressure in the tank. n = PV = (1)(46) and (1)(12) RT (.0821)(298) (.0821)(298) n O 2 = 1.9 mol n He = 0.49 mol

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In the tank, V= 5.0 L P O2 =nRT / V = (1.9)(.0821)(298) / 5.0 = 9.3 atm P he = (0.49)(.0821)(298)/ 5.0 = 2.4 atm P total = = 11.7 atm

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Homework #1 p223ff 8, 12, 15, 17, 18, 30

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Stoichiometry and Gases A sample of solid potassium chlorate decomposes by the equation 2 KClO 3 (s) -----> 2 KCl (s) + 3 O 2 (g) If 650 mL of oxygen at 22 o C and 754 torr is collected over water, how much KClO 3 decomposed? P H2O = 21 torr (from table of vapor pressure) P O2 = = 733 torr / 760 = atm n O2 = PV /RT = (.964)(.650) / (.0821)(295) = mol O 2 | 2 mol KClO 3 | g KClO 3 = 2.11 g | 3 mol O 2 | 1 mol KClO 3 KClO 3

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Homework #2 p226ff 32, 35, 38, 39, 41, 42, 44

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Kinetic Molecular Theory 1.Volume of individual particles is zero. 2.Collisions of particles with container walls cause pressure exerted by gas. 3.Particles exert no forces on each other. 4.Average kinetic energy Kelvin temperature of a gas.

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The Meaning of Temperature Kelvin temperature is an index of the random motions of gas particles (higher T means greater motion.)

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Effusion: describes the passage of gas into an evacuated chamber. Diffusion: describes the mixing of gases. The rate of diffusion is the rate of gas mixing.

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Diffusion Rates

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Effusion: Diffusion:

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Find the ratio of the effusion rates of UF 6 versus H 2. Rate H 2 = MM UF6 = 352 =13.2 Rate UF 6 MM H2 2

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Homework #3 p228ff 53, 54, 68, 70, 72, 76 #4 p229ff 77, 89, 91, 92 Super XCR #97

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Real Gases Must correct ideal gas behavior when at high pressure (smaller volume) and low temperature (attractive forces become important).

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Real Gases corrected pressure corrected volume P ideal V ideal

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