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# q1t.key - Dr Zellmer Time 30 mins Chemistry 125 Winter...

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Dr. Zellmer Chemistry 125 Tuesday Time: 30 mins Winter Quarter 2012 January 24, 2012 Quiz I Name KEY Rec. TA/time 1. (6 pts) Consider a steel tank with a volume of 68.00 L that can withstand a pressure of 2295 lb/in 2 . A sample of O 2 gas has a volume of 9841 L at STP. If the O 2 gas is put in the steel tank at 33.0°C will the tank explode? (1 atm = 760 mmHg = 760 torr = 101.325 kPa = 14.7 lb/in 2 ) STP => Standard Temp. & Pressure: T = 0°C (273.15K) & P = 1 atm Since P, V & T all change use the Combined Gas Law: P 2 V 2 P 1 V 1 ------- = ------- T MUST be in kelvin T 2 T 1 P 1 = 1 atm V 1 = 9841 L T 1 = 0.00°C + 273.15 = 273.15 K P 2 = ? V 2 = 68.00 L T 2 = 33.0°C + 273.15 = 306. 1 5 K P 1 V 1 T 2 V 1 T 2 9841 L 306. 1 5 K P 2 = ----------- = (----)(----)P 1 = (------------)(-------------) 1 atm = 162. 2 04 atm = 162.2 atm V 2 T 1 V 2 T 1 68.00 L 273.15 K Compare this to the pressure the tank can withstand before exploding, 2495 lb/in 2 . 14.7 lb/in 2 ? lb/in 2 = 162. 2 04 atm × -------------- = 238 4 .3988 lb/in 2 = 238 4 lb/in 2 1 atm Since this pressure is greater than the 2295 lb/in 2 the tank can withstand, the tank WILL explode. Copyright R. J. Zellmer, January 24, 2012

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2. (5 pts) You have both He and Ar at 500°C. Explain (no calculations necessary) using Kinetic Molecular Theory and the equations for the root-mean-square (rms) speed, u, and average kinetic energy, how the rms speeds and average kinetic energies compare. (At. Wts.: He = 4.00, Ar =39.95) KE avg = ½ m u 2 The average KE of a single “average” particle moving with the rms speed m = mass of the particle u = rms speed (type of average speed) KE total = 3/2 RT Total average KE per mole. Note it depends only on T and is directly proportional to T (in Kelvin). 3RT R = 8.314 J/mol C K T MUST be in kelvin : = (-------) 1/2 = molecular wt (molar mass in kg/mol if you want to calc : ) One can see from the eqn for the rms speed, u, it is proportional to the square root of the temp so it increases as temp. inc. When T doubles the speed doesn’t double but does inc. (by the sq. root of 2). The speed is also inversely related to the molar mass (sq. root of the molar mass). This means a larger molecule will move slower than a smaller one at the same temp. Since Ar is bigger than He and they are both at the same temp. the Ar atoms will be moving slower. Ar moves slower than He at the same temp. because Ar is bigger (more mass). The average KE is the SAME for the two atoms since their temp is the same. KE avg is directly prop.
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