# 28 - Topics for the day Administrative stuff Gas collisions...

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Topics for the day Administrative stuff Gas collisions Real gases Administrative stuff Homework Quest HW3 due Friday (10 am, 4/2) ALEKS objective ±ve due in two weeks time (10 am, 4/12) ALEKS How to choose which topics to work on when you have a growing backlog of topics you haven’t done yet. Homework clariFcation Quest HW3, Question 8 (everyone’s numbers are different) A sample of oxygen is collected in a 372 mL container over water at 15˚C and the barometer reads 776 torr. What volume would the dry gas occupy at 788 torr and 15˚C? Water’s partial pressure at 15˚C is 12.8 torr. This is an example of an ideal gas law “comparison” problem. Write a version of the ideal gas law for each of the two situations being compared, putting all variables on one side, constants on the other side. Equate and solve. The difFculty is in understanding the description of each situation: Situation one is collecting O 2 over water, so the total pressure in the container is due to the partial pressure of O 2 and the partial pressure of water vapor. Situation two is taking the “dry” gas that was just collected (i.e., just the O 2 , not the water) and seeing what volume it would occupy at the given T and p. No partial pressures involved here. 1 2 3

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Where were we? Effusion is the movement of a gas through a tiny hole from an area of high pressure into an area of low pressure. For two gases effusing under the same conditions, and for the same gas effusing at two different temperatures: The kinetic molecular theory of gases is a model that can quantitatively predict the gas laws that were initially derived from experiments. You should know the assumptions of this theory and be able to visualize what a gas looks like, as explained by this theory. Effusion rate (gas 2) Effusion rate (gas 1) M 1 M 2 = Effusion rate (T 2 ) Effusion rate (T 1 ) T 2 T 1 = For the same gas effusing at two different temperatures: The average kinetic energy of a molecule in a gas is directly proportional to the temperature of that gas. KE avg = RT 3 2 where M is the molecular weight in kg The root mean square speed of a molecule is one way to talk about an average speed: 3RT M v rms = v 2 = The peak of the velocity distribution moves higher for higher temperature. The width of the velocity distribution increases for
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28 - Topics for the day Administrative stuff Gas collisions...

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