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c102_chapter10_lecture_notes_v2 - Pressure Pressure is...

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1 Pressure Pressure is defined as force per unit area. Pressure is measured with a device called a barometer. A mercury barometer uses the weight of a column of Hg to determine the pressure of gas pushing on a reservoir— atmospheric pressure corresponds to a column height of 760 mm. Pressure may be measured in a number of different units: atmosphere (atm): barometric pressure at sea level Torr: mm of Hg—comes from use of Hg barometers psi: pounds per square inch Pascal (Pa): official SI units for pressure—1 pascal = 1 N m -2 Pressure Units Pressure conversion factors: 760 Torr = 1 atm 760 mm Hg = 1 atm 14.7 psi = 1 atm 101,325 Pa = 1 atm Pressure Units Behavior of Gases Gas-phase molecules and atoms are free to move about their container—they fill the entire volume of the container unlike a liquid or a solid. Molecular Speed and Energy Gas molecules travel at a range of speeds —some molecules move much faster than others. The average speed of a gas depends on its molar mass—the lighter the mass, the faster the average speed. Molecular Speed and Energy Average speed can be defined several ways for molecules: The most probable speed corresponds to the speed at the maximum in a plot of molecules vs speed—if we could measure the speed of individual gas molecules, more of them would have this value than any other value.
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2 Molecular Speed and Energy Kinetic energy is given by E T = ½ mu 2 m = mass u = velocity (speed) m H2 = (2.0158 g mol -1 ) / (6.022 x 10 23 H 2 mol -1 ) = 3.347 x 10 -24 g = 3.347 x 10 -27 kg u mp = 1.57 x 10 3 m s -1 E T = ½ (3.347 x 10 -27 kg) (1.57 x 10 3 m s -1 ) 2 = 4.13 x 10 -21 kg m 2 s -2 = 4.13 x 10 -21 J Molecular Speed and Energy u mp (CH 4 ) = 557 m s -1 E T (CH 4 ) = ½ (2.664 x 10 -26 kg) (557 m s -1 ) 2 = 4.13 x 10 -21 J u mp (CO 2 ) = 337 m s -1 E T (CO 2 ) = ½ (7.308 x 10 -26 kg) (337 m s -1 ) 2 = 4.15 x 10 -21 J Even though the three gases (H 2 , CH 4 , and CO 2 ) have different speeds, they all possess the same amount of kinetic energy.
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