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q1r.key

# q1r.key - Dr Zellmer Time 30 mins Chemistry 125 Winter...

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Dr. Zellmer Chemistry 125 Thursday Time: 30 mins Winter Quarter 2012 January 26, 2012 Quiz I Name KEY Rec. TA/time 1. (6 pts) Consider a steel tank with a volume of 10.0 L that can withstand a pressure of 30.0 lb/in 2 . A sample of He gas has a volume of 50.0 L at STP. If the He gas is put in the steel tank, can liquid nitrogen (boiling point of -196°C) be used to keep the tank from exploding? (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 Need to find T at which the pressure will be below or equal to 45.0 lb/in 2 . P 1 = 1.00 atm V 1 = 50.0 L T 1 = 0.00°C + 273.15 = 273.15 K 1 atm P 2 = 30.0 lb/in 2 × -------------- = 2.0 4 0 atm V 2 = 10.0 L T 2 = ? 14.7 lb/in 2 P 2 V 2 T 1 P 2 V 2 2.0 4 0 atm 10.0 L T 2 = ---------- = (-----)(-----)T 1 = (-------------)(---------) 273.15 K = 11 1 .489 K P 1 V 1 P 1 V 1 1.00 atm 50.0 L T 2 = 11 1 .489 K - 273.15 = & 161 .66°C = & 162°C Since N 2 boils at & 196 °C it can be used to cool the tank to & 162°C . The tank will NOT explode. Could also solve for P 2 using T 2 = -196°C and then compare P 2 to 30.0 lb/in 2 . (P 2 = 1.41 atm = 20.8 lb/in 2 ) Copyright R. J. Zellmer, January 26, 2012

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3. (5 pts) If the temperature of a gas increases from 200.0 K to 800.0 K what happens to the average kinetic energy of the gas particles? Be specific and quantitative using Kinetic Molecular Theory to explain. Also , sketch a qualitative picture of the distribution of speeds at the two temperatures (i.e. sketch what the two curves look like relative to each other). KE avg = ½ m : 2 The average KE of a single “average” particle moving with the rms speed m = mass of the particle : = 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). The average kinetic energy is directly proportional to the absolute temperature ( Kelvin temp.). KE avg % T (T in Kelvin) This means if the temperature increases then the average KE will increase. More specifically, if the temperature quadruples (200.0 K to 800.0 K) the KE avg quadruples (and vice versa), they are directly proportional. For 1 mole of a gas the average total KE per mole is equal to (3/2)RT.
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